music21_rs/meter/

sequence.rs

//! How a bar divides: music21's `MeterTerminal` and `MeterSequence`.
//!
//! A terminal is a span of a bar written as a ratio, with a weight. A
//! sequence is a terminal that is made of other terminals, so a bar of `4/4`
//! partitioned in two is `{1/2+1/2}`, and either half may be partitioned
//! again. music21 hangs four of these off every time signature — the beats,
//! the beams, the accents and what is displayed — and this is the part of it
//! that is arithmetic rather than notation.
//!
//! What a sequence may be partitioned into is not free: music21 keeps a list
//! of the ways each meter is conventionally divided, in priority order, and
//! partitioning by a count takes the first of those with that many parts.
//! That is why `5/8` in two is `{2/8+3/8}` and not `{2.5/8+2.5/8}`.

use crate::defaults::{FloatType, UnsignedIntegerType};
use crate::error::{Error, Result};

/// How close two quarter lengths must be to count as the same boundary.
const OFFSET_TOLERANCE: FloatType = 1e-9;

/// The denominators music21 will write a meter in.
const VALID_DENOMINATORS: [UnsignedIntegerType; 8] = [1, 2, 4, 8, 16, 32, 64, 128];

/// How an offset is matched against the boundaries of a level: music21's
/// `align` argument to `offsetToDepth`.
#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub enum OffsetAlign {
    /// Move the offset back to the start of the finest part holding it, then
    /// count the levels beginning there. music21's default.
    #[default]
    Quantize,
    /// Count only the levels whose part begins exactly at the offset.
    Start,
    /// Count the levels whose part ends exactly at the offset.
    End,
}

/// One span of a bar: a ratio, and how strongly it is felt.
#[derive(Clone, Debug, PartialEq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub struct MeterTerminal {
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
    weight: FloatType,
    /// What this span is itself divided into, if anything. A terminal with
    /// parts is music21's `MeterSequence`; one without is a leaf.
    parts: Vec<MeterTerminal>,
    /// Whether this was written with its numerators summed over one
    /// denominator, `3+2/8`, rather than part by part, `3/8+2/8`: music21's
    /// `summedNumerator`.
    #[cfg_attr(feature = "serde", serde(default))]
    summed_numerator: bool,
}

impl MeterTerminal {
    /// A span of `numerator/denominator`, undivided, weighing one.
    pub fn new(numerator: UnsignedIntegerType, denominator: UnsignedIntegerType) -> Result<Self> {
        if numerator == 0 {
            return Err(Error::Meter(
                "a meter terminal numerator must be non-zero".to_string(),
            ));
        }
        if denominator == 0 {
            return Err(Error::Meter(
                "a meter terminal denominator must be non-zero".to_string(),
            ));
        }
        Ok(Self {
            numerator,
            denominator,
            weight: 1.0,
            parts: Vec::new(),
            summed_numerator: false,
        })
    }

    /// The same, read from `"3/8"`.
    pub fn from_ratio_string(ratio: &str) -> Result<Self> {
        let (numerator, denominator) = split_ratio(ratio)?;
        Self::new(numerator, denominator)
    }

    /// A span written as parts joined by `+`, each part a ratio: music21's
    /// `MeterSequence('2/4+2/4')`, which is a bar of four quarters divided in
    /// two. A part written as a bare numerator takes the denominator of the
    /// next part that gives one, so `3+2/8` is three eighths and two eighths.
    ///
    /// The span itself is written over the denominator its parts share, or
    /// the least one they all divide into: `3/4+2/4+1/8` is `11/8`.
    ///
    /// A span written with a bare numerator is one whose numerators were
    /// summed, and says so: [`Self::summed_numerator`].
    pub fn from_partition_string(written: &str) -> Result<Self> {
        let mut parts = Vec::new();
        let tokens: Vec<&str> = written.split('+').map(str::trim).collect();
        // Read from the right, so a bare numerator can take the denominator
        // of the part that follows it.
        let mut denominators: Vec<UnsignedIntegerType> = vec![0; tokens.len()];
        let mut carried = None;
        for (index, token) in tokens.iter().enumerate().rev() {
            if let Some((_, denominator)) = token.split_once('/') {
                carried = Some(parse_number(denominator)?);
            }
            denominators[index] = carried.ok_or_else(|| {
                Error::Meter(format!(
                    "cannot read a denominator from {token:?} in {written:?}"
                ))
            })?;
        }

        parts.reserve(tokens.len());
        for (token, denominator) in tokens.iter().zip(denominators) {
            let numerator = parse_number(token.split('/').next().unwrap_or(token))?;
            parts.push(Self::new(numerator, denominator)?);
        }
        let mut span = Self::from_parts(parts, written)?;
        span.summed_numerator = tokens.iter().any(|token| !token.contains('/'));
        Ok(span)
    }

    /// Whether this span was written with its numerators summed over one
    /// denominator, as `3+2/8` is and `3/8+2/8` is not: music21's
    /// `summedNumerator`. The two are the same parts; what differs is how
    /// the meter is printed, and that a bar written this way is beamed in the
    /// parts it was written in.
    #[must_use]
    pub fn summed_numerator(&self) -> bool {
        self.summed_numerator
    }

    /// Says whether the numerators are written summed.
    pub fn set_summed_numerator(&mut self, summed: bool) {
        self.summed_numerator = summed;
    }

    /// A span made of the parts given, which is what they come to: music21's
    /// `MeterSequence` given a list of spans. The span is written over the
    /// denominator its parts share, or the least one they all divide into.
    ///
    /// One part alone is that part, undivided, as music21 reads `'4/4'`.
    pub fn from_parts_given(parts: Vec<Self>) -> Result<Self> {
        Self::from_parts(parts, "a list of spans")
    }

    fn from_parts(mut parts: Vec<Self>, written: &str) -> Result<Self> {
        if parts.len() == 1 {
            // One part joined to nothing is just that span, as music21 reads
            // `MeterSequence('4/4')` -- the sequence wrapping is the caller's.
            return Ok(parts.remove(0));
        }
        Self::joining(parts, written)
    }

    /// A span written over what its parts come to, divided into them: the
    /// ratio is read off the parts rather than given, which is music21's
    /// `_updateRatio`.
    fn joining(parts: Vec<Self>, written: &str) -> Result<Self> {
        let shared = parts
            .iter()
            .map(Self::denominator)
            .reduce(num::integer::lcm)
            .ok_or_else(|| Error::Meter(format!("{written:?} is no meter at all")))?;
        let total = parts
            .iter()
            .map(|part| part.numerator() * (shared / part.denominator()))
            .sum();
        let mut span = Self::new(total, shared)?;
        *span.parts_mut() = parts;
        Ok(span)
    }

    /// The numerator this span is written with.
    #[must_use]
    pub fn numerator(&self) -> UnsignedIntegerType {
        self.numerator
    }

    /// The denominator this span is written with.
    #[must_use]
    pub fn denominator(&self) -> UnsignedIntegerType {
        self.denominator
    }

    /// How long this span is in quarter notes.
    #[must_use]
    pub fn quarter_length(&self) -> FloatType {
        FloatType::from(self.numerator) * (4.0 / FloatType::from(self.denominator))
    }

    /// Every index it takes to reach the terminal sounding at an offset:
    /// music21's `offsetToAddress`. Its length is how deep that terminal
    /// lies, so a span nothing divides answers one index.
    pub fn offset_to_address(&self, offset: FloatType) -> Result<Vec<usize>> {
        let length = self.quarter_length();
        if offset.is_nan() || offset < 0.0 || offset >= length {
            return Err(Error::Meter(format!(
                "cannot access from qLenPos {}",
                super::offset_repr(offset)
            )));
        }
        let index = self.offset_to_index(offset)?;
        let mut address = vec![index];
        let (start, _) = self.offset_to_span(offset, false)?;
        let part = &self.parts[index];
        if !part.is_empty() {
            address.extend(part.offset_to_address(offset - start)?);
        }
        Ok(address)
    }

    /// How strongly this span is felt: music21's `weight`.
    ///
    /// A span with parts weighs what its parts weigh together, so weight is
    /// kept on the terminals and read back up the tree.
    #[must_use]
    pub fn weight(&self) -> FloatType {
        if self.parts.is_empty() {
            self.weight
        } else {
            self.parts.iter().map(Self::weight).sum()
        }
    }

    /// Sets how strongly this span is felt.
    ///
    /// A span with parts shares the weight out among them in proportion to
    /// how long each is, as music21 does: the four quarters of a bar weighing
    /// one take a quarter each, where `{3/4+1/4}` takes three quarters and a
    /// quarter.
    pub fn set_weight(&mut self, weight: FloatType) {
        self.weight = weight;
        let whole = self.quarter_length();
        if self.parts.is_empty() || whole == 0.0 {
            return;
        }
        for part in &mut self.parts {
            let share = part.quarter_length() / whole;
            part.set_weight(weight * share);
        }
    }

    /// What this span is divided into. Empty where it is a leaf.
    #[must_use]
    pub fn parts(&self) -> &[MeterTerminal] {
        &self.parts
    }

    /// The same, to be changed.
    pub fn parts_mut(&mut self) -> &mut Vec<MeterTerminal> {
        &mut self.parts
    }

    /// How many parts this span has at its top level, music21's `len`.
    #[must_use]
    pub fn len(&self) -> usize {
        self.parts.len()
    }

    /// Whether this span is divided at all.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.parts.is_empty()
    }

    /// How many levels of division this span has: music21's `depth`. A leaf
    /// is nought.
    #[must_use]
    pub fn depth(&self) -> usize {
        self.parts
            .iter()
            .map(|part| 1 + part.depth())
            .max()
            .unwrap_or(0)
    }

    /// Every leaf of this span, in order, which is what `flatten` is made
    /// of: music21's `_getFlatList`.
    #[must_use]
    pub fn flattened(&self) -> Vec<MeterTerminal> {
        if self.parts.is_empty() {
            return vec![self.clone()];
        }
        self.parts
            .iter()
            .flat_map(MeterTerminal::flattened)
            .collect()
    }

    /// This span divided into all of its leaves at once, however deeply
    /// each was divided: music21's `flatten`.
    pub fn flatten(&self) -> Result<Self> {
        // Written over what the leaves come to, so a `4/4` bar in eighths
        // flattens to `8/8`, as music21's does.
        Self::joining(self.flattened(), "a flattened span")
    }

    /// What each leaf of this span weighs: music21's `flatWeight`.
    #[must_use]
    pub fn flat_weights(&self) -> Vec<FloatType> {
        self.flattened().iter().map(MeterTerminal::weight).collect()
    }

    /// Whether another span is written over the same ratio: music21's
    /// `ratioEqual`. Two spans may weigh differently and be divided
    /// differently and still be the same meter.
    #[must_use]
    pub fn ratio_equal(&self, other: &Self) -> bool {
        self.numerator == other.numerator && self.denominator == other.denominator
    }

    /// Divides this span into the parts named, as `["2/8", "3/8"]`.
    ///
    /// The parts must come to what this span already is; a partition that
    /// lengthened or shortened the bar would not be one.
    pub fn partition_by_parts(&mut self, parts: &[&str]) -> Result<()> {
        let mut built = Vec::with_capacity(parts.len());
        for part in parts {
            built.push(MeterTerminal::from_ratio_string(part)?);
        }
        let total: FloatType = built.iter().map(MeterTerminal::quarter_length).sum();
        if (total - self.quarter_length()).abs() > OFFSET_TOLERANCE {
            return Err(Error::Meter(format!(
                "Cannot set partition by {}",
                python_list(parts)
            )));
        }
        let held = self.weight();
        self.parts = built;
        self.set_weight(held);
        Ok(())
    }

    /// Divides this span into `count` parts, the way music21 divides it.
    ///
    /// The first of [`Self::partition_options`] with that many parts wins. A
    /// count nothing divides into leaves the first option standing, which is
    /// music21's `loadDefault`.
    pub fn partition_by_count(&mut self, count: usize, load_default: bool) -> Result<()> {
        let options = self.partition_options();
        let chosen = options.iter().find(|option| option.len() == count);
        let chosen = match chosen {
            Some(option) => option.clone(),
            None => {
                if !load_default || options.is_empty() {
                    return Err(Error::Meter(format!(
                        "Cannot set partition by {count} ({}/{})",
                        self.numerator, self.denominator
                    )));
                }
                options[0].clone()
            }
        };
        let borrowed: Vec<&str> = chosen.iter().map(String::as_str).collect();
        self.partition_by_parts(&borrowed)
    }

    /// Divides this span by a list of numerators, as `[3, 1]` for `{3/4+1/4}`.
    ///
    /// music21 reads the list against this span's own denominator where the
    /// numerators come to its numerator, and against a finer one where they
    /// come to a multiple of it — `[1, 1, 1, 1]` of a `2/4` bar is four
    /// eighths, not four quarters.
    ///
    /// Failing both, it looks for a division of its own whose numerators are
    /// the ones asked for, which is how `[1, 1]` of a `4/4` bar comes out as
    /// two halves and `[1]` as a whole.
    pub fn partition_by_list(&mut self, numerators: &[UnsignedIntegerType]) -> Result<()> {
        if numerators.is_empty() {
            return Err(Error::Meter(
                "cannot partition a meter into nothing".to_string(),
            ));
        }
        let total: UnsignedIntegerType = numerators.iter().sum();
        for multiple in 1..=8 {
            if total != self.numerator * multiple {
                continue;
            }
            let denominator = self.denominator * multiple;
            let parts: Vec<String> = numerators
                .iter()
                .map(|numerator| format!("{numerator}/{denominator}"))
                .collect();
            let borrowed: Vec<&str> = parts.iter().map(String::as_str).collect();
            return self.partition_by_parts(&borrowed);
        }
        let asked: Vec<UnsignedIntegerType> = numerators.to_vec();
        let matching = self.partition_options().into_iter().find(|option| {
            option
                .iter()
                .map(|part| {
                    part.split('/')
                        .next()
                        .and_then(|numerator| numerator.parse().ok())
                })
                .collect::<Option<Vec<UnsignedIntegerType>>>()
                == Some(asked.clone())
        });
        if let Some(option) = matching {
            let borrowed: Vec<&str> = option.iter().map(String::as_str).collect();
            return self.partition_by_parts(&borrowed);
        }
        Err(Error::Meter(format!(
            "Cannot set partition by {} ({}/{})",
            python_list(numerators),
            self.numerator,
            self.denominator
        )))
    }

    /// Divides this span into `count` parts and leaves those parts divided
    /// as they were: music21's `MeterTerminal.subdivide`.
    pub fn subdivide(&mut self, count: usize) -> Result<()> {
        self.partition_by_count(count, false)
    }

    /// Divides every part of this span again, the way music21 divides it
    /// when nobody has said how: music21's `subdividePartitionsEqual`.
    ///
    /// `divisions` says how many parts each part becomes; `None` asks for
    /// the division each part conventionally takes — two for a part written
    /// in a binary numerator, three for one written in three, and threes for
    /// the compound numerators. A part nothing divides into is an error,
    /// which is what music21 raises and what a meter written in notes
    /// shorter than a 128th is forgiven.
    pub fn subdivide_partitions_equal(&mut self, divisions: Option<usize>) -> Result<()> {
        for part in &mut self.parts {
            let count = match divisions {
                Some(count) => count,
                None => match part.numerator {
                    1 | 2 | 4 | 8 | 16 | 32 | 64 => 2,
                    3 => 3,
                    6 | 9 | 12 | 15 | 18 | 21 | 24 | 27 => (part.numerator / 3) as usize,
                    other => other as usize,
                },
            };
            part.partition_by_count(count, false)?;
        }
        Ok(())
    }

    /// The part a path of indices names, and the parts it holds.
    fn part_at(&self, path: &[usize]) -> &Self {
        path.iter().fold(self, |span, index| &span.parts[*index])
    }

    fn part_at_mut(&mut self, path: &[usize]) -> &mut Self {
        path.iter()
            .fold(self, |span, index| &mut span.parts[*index])
    }

    /// Divides the parts of every span named, and answers where those parts
    /// now are: music21's `_subdivideNested`, which is one level of
    /// [`Self::subdivide_nested_hierarchy`].
    fn subdivide_parts_of(
        &mut self,
        spans: &[Vec<usize>],
        divisions: Option<usize>,
    ) -> Result<Vec<Vec<usize>>> {
        let mut deeper = Vec::new();
        for path in spans {
            let span = self.part_at_mut(path);
            span.subdivide_partitions_equal(divisions)?;
            for index in 0..span.len() {
                let mut child = path.clone();
                child.push(index);
                deeper.push(child);
            }
        }
        Ok(deeper)
    }

    /// Nests this span down to a depth: music21's `subdivideNestedHierarchy`.
    ///
    /// The whole span becomes one part, that part is divided by two, by three
    /// or by its own numerator, and every division after that is by two or
    /// three again — so a bar of four quarters at depth two is two halves of
    /// two quarters each. Whatever partitions the span had are gone.
    ///
    /// `normalize_denominators` keeps the parts of a level written over one
    /// denominator: where they differ, the longer ones are divided again
    /// until they agree, which is why six eighths come out as sixteenths.
    ///
    /// ```
    /// use music21_rs::meter::MeterTerminal;
    ///
    /// let mut bar = MeterTerminal::new(4, 4)?;
    /// bar.subdivide_nested_hierarchy(2, None, true)?;
    /// assert_eq!(bar.to_string(), "{{{1/4+1/4}+{1/4+1/4}}}");
    /// # Ok::<(), music21_rs::Error>(())
    /// ```
    pub fn subdivide_nested_hierarchy(
        &mut self,
        depth: usize,
        first_partition_form: Option<UnsignedIntegerType>,
        normalize_denominators: bool,
    ) -> Result<()> {
        self.partition_by_count(1, false)?;
        if depth == 0 {
            return Ok(());
        }
        let form = first_partition_form.unwrap_or(self.numerator);
        let first = match form {
            1 | 2 | 4 | 8 | 16 | 32 => 2,
            3 => 3,
            other => other as usize,
        };
        self.subdivide_partitions_equal(Some(first))?;
        self.subdivide_nested_below_the_first(depth, normalize_denominators)
    }

    /// The same, with the first division taken from another sequence rather
    /// than from a count: music21's `subdivideNestedHierarchy` handed a
    /// `MeterSequence` as its `firstPartitionForm`.
    ///
    /// This is how a bar whose beats are not all one length is nested — the
    /// five eighths of `3+2/8` divide first into a three and a two, which no
    /// count can say. The form has to come to the same length as this span.
    ///
    /// ```
    /// use music21_rs::meter::MeterTerminal;
    ///
    /// let beats = MeterTerminal::from_partition_string("3/8+2/8")?;
    /// let mut bar = MeterTerminal::new(5, 8)?;
    /// bar.subdivide_nested_hierarchy_by(2, &beats, true)?;
    /// assert_eq!(bar.to_string(), "{{{1/8+1/8+1/8}+{1/8+1/8}}}");
    /// # Ok::<(), music21_rs::Error>(())
    /// ```
    pub fn subdivide_nested_hierarchy_by(
        &mut self,
        depth: usize,
        first_partition_form: &Self,
        normalize_denominators: bool,
    ) -> Result<()> {
        self.partition_by_count(1, false)?;
        if depth == 0 {
            return Ok(());
        }
        let ratio = |span: &Self| format!("{}/{}", span.numerator, span.denominator);
        let written: Vec<String> = if first_partition_form.parts.is_empty() {
            vec![ratio(first_partition_form)]
        } else {
            first_partition_form.parts.iter().map(ratio).collect()
        };
        let borrowed: Vec<&str> = written.iter().map(String::as_str).collect();
        self.parts[0].partition_by_parts(&borrowed)?;
        self.subdivide_nested_below_the_first(depth, normalize_denominators)
    }

    /// Every level of a nested hierarchy below the first division, which is
    /// by two or three however the first was made.
    fn subdivide_nested_below_the_first(
        &mut self,
        depth: usize,
        normalize_denominators: bool,
    ) -> Result<()> {
        let mut frontier = vec![vec![0]];
        for _ in 1..depth {
            frontier = self.subdivide_parts_of(&frontier, None)?;
            if !normalize_denominators {
                continue;
            }
            // Where a level is written over more than one denominator, the
            // longer parts are divided again until it is written over one.
            loop {
                let denominators: Vec<UnsignedIntegerType> = frontier
                    .iter()
                    .map(|path| self.part_at(path).denominator())
                    .collect();
                let Some(shortest) = denominators.iter().min().copied() else {
                    break;
                };
                if denominators
                    .iter()
                    .all(|denominator| *denominator == shortest)
                {
                    break;
                }
                let mut evened = Vec::new();
                for path in &frontier {
                    if self.part_at(path).denominator() == shortest {
                        evened.extend(self.subdivide_parts_of(std::slice::from_ref(path), None)?);
                    } else {
                        evened.push(path.clone());
                    }
                }
                frontier = evened;
            }
        }
        Ok(())
    }

    /// The ways music21 conventionally divides this meter, in the order it
    /// prefers them: music21's `getPartitionOptions`.
    #[must_use]
    pub fn partition_options(&self) -> Vec<Vec<String>> {
        let mut options = division_options_algorithmic(self.numerator, self.denominator);
        options.extend(division_options_preset(self.numerator, self.denominator));
        let mut seen = Vec::new();
        options.retain(|option| {
            if option.is_empty() || seen.contains(option) {
                return false;
            }
            seen.push(option.clone());
            true
        });
        options
    }

    /// The terminals at one level of this sequence: music21's `getLevelList`.
    ///
    /// A part that is not divided is taken as it stands. A part that is gets
    /// recursed into while there are levels left to descend, and at the level
    /// asked for is either kept whole or, when `flat`, flattened into a
    /// single terminal carrying that part's own weight.
    #[must_use]
    pub fn level_list(&self, level: usize, flat: bool) -> Vec<MeterTerminal> {
        let mut out = Vec::new();
        for part in &self.parts {
            if part.parts.is_empty() {
                out.push(part.clone());
            } else if level > 0 {
                out.extend(part.level_list(level - 1, flat));
            } else if flat {
                let mut flattened = part.clone();
                flattened.parts.clear();
                out.push(flattened);
            } else {
                out.push(part.clone());
            }
        }
        out
    }

    /// One level of this sequence as a sequence of its own: music21's
    /// `getLevel`.
    pub fn level(&self, level: usize, flat: bool) -> Result<Self> {
        let mut out = Self::new(self.numerator, self.denominator)?;
        out.weight = self.weight;
        out.parts = self.level_list(level, flat);
        Ok(out)
    }

    /// Where each terminal of a level starts and ends, in quarter lengths
    /// from the start of this span: music21's `getLevelSpan`.
    #[must_use]
    pub fn level_span(&self, level: usize) -> Vec<(FloatType, FloatType)> {
        let mut spans = Vec::new();
        let mut position = 0.0;
        for part in self.level_list(level, true) {
            let end = position + part.quarter_length();
            spans.push((position, end));
            position = end;
        }
        spans
    }

    /// How many levels of this sequence start at an offset: music21's
    /// `offsetToDepth`.
    ///
    /// A level counts when one of its parts begins where the offset does.
    /// Under [`OffsetAlign::Quantize`] the offset is first moved back to the
    /// start of the finest part holding it, so an offset inside a part still
    /// counts the levels that part begins.
    pub fn offset_to_depth(&self, offset: FloatType, align: OffsetAlign) -> Result<usize> {
        let length = self.quarter_length();
        if offset.is_nan() || offset < 0.0 || offset >= length {
            return Err(Error::Meter(format!(
                "cannot access from qLenPos {}",
                super::offset_repr(offset)
            )));
        }
        let depth = self.depth();
        if depth == 0 {
            return Ok(0);
        }
        let finest = self.level(depth - 1, true)?;
        let index = finest.offset_to_index(offset)?;
        let spans = self.level_span(depth - 1);
        let position = match align {
            OffsetAlign::Quantize => spans[index].0,
            OffsetAlign::Start | OffsetAlign::End => offset,
        };
        let mut score = 0;
        for level in 0..depth {
            for (start, end) in self.level_span(level) {
                let boundary = match align {
                    OffsetAlign::Start | OffsetAlign::Quantize => start,
                    OffsetAlign::End => end,
                };
                if (boundary - position).abs() < OFFSET_TOLERANCE {
                    score += 1;
                }
            }
        }
        Ok(score)
    }

    /// Weighs the terminals of a level, looping the weights given over them:
    /// the write half of music21's `setAccentWeight`.
    ///
    /// A level with no terminals, or no weights to give it, is an error
    /// rather than a silent nothing.
    pub fn set_level_weight(&mut self, level: usize, weights: &[FloatType]) -> Result<()> {
        if weights.is_empty() {
            return Err(Error::Meter(
                "a weight has to be given to weigh a level with".to_string(),
            ));
        }
        let mut index = 0;
        Self::weigh(&mut self.parts, level, weights, &mut index);
        if index == 0 {
            return Err(Error::Meter(format!(
                "this meter has no level {level} to weigh"
            )));
        }
        Ok(())
    }

    /// Walks the terminals of a level in order, weighing each in turn.
    fn weigh(parts: &mut [MeterTerminal], level: usize, weights: &[FloatType], index: &mut usize) {
        for part in parts.iter_mut() {
            if part.parts.is_empty() || level == 0 {
                part.weight = weights[*index % weights.len()];
                *index += 1;
            } else {
                Self::weigh(&mut part.parts, level - 1, weights, index);
            }
        }
    }

    /// Whether every part of a level is the same ratio: music21's
    /// `isUniformPartition`.
    #[must_use]
    pub fn is_uniform_partition(&self, depth: usize) -> bool {
        let mut numerator = None;
        let mut denominator = None;
        for part in self.level_list(depth, false) {
            if *numerator.get_or_insert(part.numerator) != part.numerator
                || *denominator.get_or_insert(part.denominator) != part.denominator
            {
                return false;
            }
        }
        true
    }

    /// The parts written out without the braces around them: music21's
    /// `partitionDisplay`, so a bar of `2/4+6/8` reads as it was written.
    #[must_use]
    pub fn partition_display(&self) -> String {
        self.parts
            .iter()
            .map(MeterTerminal::to_string)
            .collect::<Vec<String>>()
            .join("+")
    }

    /// This span divided into `count` parts, as a new sequence: music21's
    /// `subdivideByCount`. The weight of this span goes with it.
    pub fn subdivide_by_count(&self, count: usize) -> Result<Self> {
        let mut out = Self::new(self.numerator, self.denominator)?;
        out.weight = self.weight;
        out.partition_by_count(count, true)?;
        Ok(out)
    }

    /// This span divided into the one span another sequence is: music21's
    /// `subdivideByOther`. The other becomes the only part of the answer, so
    /// what the other was divided into is a level further down; a span of
    /// another length is an error, since a division cannot relength the bar.
    pub fn subdivide_by_other(&self, other: &Self) -> Result<Self> {
        if (other.quarter_length() - self.quarter_length()).abs() > OFFSET_TOLERANCE {
            return Err(Error::Meter(format!("cannot subdivide by other: {other}")));
        }
        let mut out = Self::new(other.numerator, other.denominator)?;
        out.parts_mut().push(other.clone());
        Ok(out)
    }

    /// Divides this span the way another sequence is divided: music21's
    /// `partitionByOtherMeterSequence`. The two have to be written over the
    /// same ratio, and this span keeps the weight it had.
    pub fn partition_by_other(&mut self, other: &Self) -> Result<()> {
        if !self.ratio_equal(other) {
            return Err(Error::Meter(
                "Cannot set partition for unequal MeterSequences".to_string(),
            ));
        }
        let weight = self.weight();
        self.parts = other.parts.clone();
        self.set_weight(weight);
        Ok(())
    }

    /// What each terminal of a level weighs: music21's `getLevelWeight`.
    #[must_use]
    pub fn level_weight(&self, level: usize) -> Vec<FloatType> {
        self.level_list(level, true)
            .iter()
            .map(MeterTerminal::weight)
            .collect()
    }

    /// This span divided by a list of numerators, as a new sequence:
    /// music21's `subdivideByList`.
    pub fn subdivide_by_list(&self, numerators: &[UnsignedIntegerType]) -> Result<Self> {
        let mut out = Self::new(self.numerator, self.denominator)?;
        out.weight = self.weight;
        out.partition_by_list(numerators)?;
        Ok(out)
    }

    /// Which part an offset in quarter notes falls in: music21's
    /// `offsetToIndex`. An offset outside the span is an error.
    pub fn offset_to_index(&self, offset: FloatType) -> Result<usize> {
        let offset = super::offset_within(offset, self.quarter_length())?;
        let mut start = 0.0;
        for (index, part) in self.parts.iter().enumerate() {
            let end = start + part.quarter_length();
            if offset >= start - OFFSET_TOLERANCE && offset < end - OFFSET_TOLERANCE {
                return Ok(index);
            }
            start = end;
        }
        Ok(self.parts.len().saturating_sub(1))
    }

    /// Where the part an offset falls in begins and ends: music21's
    /// `offsetToSpan`. With `permit_meter_modulus` an offset past the end of
    /// the span is read within it.
    pub fn offset_to_span(
        &self,
        offset: FloatType,
        permit_meter_modulus: bool,
    ) -> Result<(FloatType, FloatType)> {
        let length = self.quarter_length();
        let offset = if permit_meter_modulus && offset >= length {
            offset.rem_euclid(length)
        } else {
            offset
        };
        let index = self.offset_to_index(offset)?;
        let mut start = 0.0;
        for (position, part) in self.parts.iter().enumerate() {
            let end = start + part.quarter_length();
            if position == index {
                return Ok((start, end));
            }
            start = end;
        }
        Ok((0.0, length))
    }

    /// The weight of the part an offset falls in: music21's `offsetToWeight`.
    pub fn offset_to_weight(&self, offset: FloatType) -> Result<FloatType> {
        let index = self.offset_to_index(offset)?;
        Ok(self
            .parts
            .get(index)
            .map_or(self.weight, MeterTerminal::weight))
    }

    /// How this span is written: `3/4` undivided, `{1/4+1/4+1/4}` divided.
    #[must_use]
    pub fn partition_string(&self) -> String {
        if self.parts.is_empty() {
            return format!("{}/{}", self.numerator, self.denominator);
        }
        let inner: Vec<String> = self
            .parts
            .iter()
            .map(MeterTerminal::partition_string)
            .collect();
        format!("{{{}}}", inner.join("+"))
    }
}

impl std::fmt::Display for MeterTerminal {
    fn fmt(&self, formatter: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        formatter.write_str(&self.partition_string())
    }
}

/// A list of parts as Python prints it, since music21 names a partition it
/// cannot set by printing the list it was handed: `['3/4', '1/8']` for the
/// written parts, `[1, 1]` for their numerators.
fn python_list<T: std::fmt::Display>(parts: &[T]) -> String {
    let quote = |part: &T| {
        let written = part.to_string();
        if written.parse::<FloatType>().is_ok() {
            written
        } else {
            format!("'{written}'")
        }
    };
    format!(
        "[{}]",
        parts.iter().map(quote).collect::<Vec<String>>().join(", ")
    )
}

/// One of the numbers a meter is written with.
fn parse_number(text: &str) -> Result<UnsignedIntegerType> {
    text.trim()
        .parse()
        .map_err(|_| Error::Meter(format!("{text:?} is not a number a meter is written with")))
}

/// `"3/8"` as the two numbers it is written with.
fn split_ratio(ratio: &str) -> Result<(UnsignedIntegerType, UnsignedIntegerType)> {
    let trimmed = ratio.trim();
    let (numerator, denominator) = trimmed.split_once('/').ok_or_else(|| {
        Error::Meter(format!(
            "a meter terminal is written numerator/denominator, not {ratio:?}"
        ))
    })?;
    let read = |part: &str, label: &str| {
        part.trim()
            .parse::<UnsignedIntegerType>()
            .map_err(|_| Error::Meter(format!("cannot read a {label} from {part:?} in {ratio:?}")))
    };
    Ok((
        read(numerator, "numerator")?,
        read(denominator, "denominator")?,
    ))
}

/// The same ratio in shorter notes, while the denominator allows: music21's
/// `divisionOptionsFractionsUpward`.
fn fractions_upward(
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
) -> Vec<String> {
    let largest = VALID_DENOMINATORS[VALID_DENOMINATORS.len() - 1];
    let mut out = Vec::new();
    if denominator >= largest {
        return out;
    }
    let (mut numerator, mut denominator) = (numerator * 2, denominator * 2);
    while denominator <= largest {
        out.push(format!("{numerator}/{denominator}"));
        numerator *= 2;
        denominator *= 2;
    }
    out
}

/// The same ratio in longer notes, while it stays whole: music21's
/// `divisionOptionsFractionsDownward`.
fn fractions_downward(
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
) -> Vec<String> {
    let smallest = VALID_DENOMINATORS[0];
    let mut out = Vec::new();
    if denominator <= smallest || !numerator.is_multiple_of(2) {
        return out;
    }
    let (mut numerator, mut denominator) = (numerator / 2, denominator / 2);
    loop {
        out.push(format!("{numerator}/{denominator}"));
        if !numerator.is_multiple_of(2) || denominator <= smallest {
            break;
        }
        numerator /= 2;
        denominator /= 2;
    }
    out
}

/// One unit at a time, then twice as many half as long: music21's
/// `divisionOptionsAdditiveMultiplesUpward`, which stops at sixteen parts or
/// at the numerator where that is larger.
fn additive_multiples_upward(
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
) -> Vec<Vec<String>> {
    let mut out = Vec::new();
    if numerator <= 1 {
        return out;
    }
    let largest = VALID_DENOMINATORS[VALID_DENOMINATORS.len() - 1];
    let limit = if numerator > 16 { numerator } else { 16 };
    let (mut denominator, mut count) = (denominator, numerator);
    while denominator <= largest && count <= limit {
        out.push(vec![format!("1/{denominator}"); count as usize]);
        denominator *= 2;
        count *= 2;
    }
    out
}

/// The bar halved, and halved again while it stays even: music21's
/// `divisionOptionsAdditiveMultiplesEvenDivision`. A `4/4` bar reads
/// `1/2+1/2`, written in the note value a half of it actually is.
fn additive_multiples_even_division(
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
) -> Vec<Vec<String>> {
    let mut out = Vec::new();
    if !numerator.is_multiple_of(2) || denominator < 2 {
        return out;
    }
    let (mut count, mut denominator) = (numerator / 2, denominator / 2);
    while denominator >= 1 && count > 1 {
        out.push(vec![format!("1/{denominator}"); count as usize]);
        if !count.is_multiple_of(2) || denominator == 1 {
            break;
        }
        denominator /= 2;
        count /= 2;
    }
    out
}

/// Even groupings in the bar's own note value: music21's
/// `divisionOptionsAdditiveMultiples`, which reads `6/4` as `3/4+3/4`.
fn additive_multiples(
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
) -> Vec<Vec<String>> {
    let mut out = Vec::new();
    if numerator <= 3 || !numerator.is_multiple_of(2) {
        return out;
    }
    let mut divisor = 2;
    while numerator.is_multiple_of(divisor) {
        let count = numerator / divisor;
        if count <= 1 {
            break;
        }
        out.push(vec![format!("{count}/{denominator}"); divisor as usize]);
        divisor *= 2;
    }
    out
}

/// A single unit split into smaller ones: music21's
/// `divisionOptionsAdditiveMultiplesDownward`, which only ever applies where
/// the numerator is one.
fn additive_multiples_downward(
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
) -> Vec<Vec<String>> {
    let largest = VALID_DENOMINATORS[VALID_DENOMINATORS.len() - 1];
    let mut out = Vec::new();
    if denominator >= largest || numerator != 1 {
        return out;
    }
    let (mut count, mut denominator) = (2usize, denominator * 2);
    while denominator <= largest {
        out.push(vec![format!("{numerator}/{denominator}"); count]);
        denominator *= 2;
        count *= 2;
    }
    out
}

/// The ways music21 divides a meter, in the order it prefers them:
/// music21's `divisionOptionsAlgo`.
///
/// The order is the behaviour, not decoration:
/// [`MeterTerminal::partition_by_count`] takes the first option of the right
/// length, so these pieces are composed in the same sequence music21
/// composes them in.
fn division_options_algorithmic(
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
) -> Vec<Vec<String>> {
    let mut options: Vec<Vec<String>> = Vec::new();

    // Compound meters divide into threes first: 6/8 is two dotted beats.
    if numerator > 3 && numerator.is_multiple_of(3) {
        options.push(vec![format!("3/{denominator}"); (numerator / 3) as usize]);
    }
    // The odd meters music21 keeps a grouping for.
    let groupings: &[&[UnsignedIntegerType]] = match numerator {
        5 => &[&[2, 3], &[3, 2]],
        7 => &[&[2, 2, 3], &[3, 2, 2], &[2, 3, 2]],
        10 => &[&[2, 2, 3, 3]],
        _ => &[],
    };
    for grouping in groupings {
        options.push(
            grouping
                .iter()
                .map(|count| format!("{count}/{denominator}"))
                .collect(),
        );
    }
    options.extend(additive_multiples_upward(numerator, denominator));
    options.extend(additive_multiples_even_division(numerator, denominator));
    options.push(vec![format!("{numerator}/{denominator}")]);
    options.extend(additive_multiples(numerator, denominator));
    options.extend(additive_multiples_downward(numerator, denominator));
    for written in fractions_downward(numerator, denominator) {
        options.push(vec![written]);
    }
    for written in fractions_upward(numerator, denominator) {
        options.push(vec![written]);
    }
    options
}

/// The divisions music21 keeps by hand because no rule produces them: only
/// the two extra readings of a five.
fn division_options_preset(
    numerator: UnsignedIntegerType,
    denominator: UnsignedIntegerType,
) -> Vec<Vec<String>> {
    if numerator != 5 {
        return Vec::new();
    }
    vec![
        vec![
            format!("2/{denominator}"),
            format!("2/{denominator}"),
            format!("1/{denominator}"),
        ],
        vec![
            format!("2/{denominator}"),
            format!("1/{denominator}"),
            format!("2/{denominator}"),
        ],
    ]
}

#[cfg(test)]
mod tests {
    use super::MeterTerminal;
    use super::OffsetAlign;

    /// music21's own `divisionOptionsAlgo(4, 4)`, in its order.
    ///
    /// The order is what decides a partition: `partition_by_count` takes the
    /// first option of the length asked for, so `4/4` in two is `1/2+1/2`
    /// and not `2/4+2/4`, which comes later in the same list.
    #[test]
    fn levels_are_read_as_music21_reads_them() {
        // Read off music21 11.0.0b9: TimeSignature("4/4").beatSequence.
        let mut bar = MeterTerminal::new(4, 4).unwrap();
        bar.partition_by_parts(&["4/4"]).unwrap();
        bar.partition_by_count(4, true).unwrap();
        bar.subdivide_partitions_equal(None).unwrap();
        assert_eq!(bar.to_string(), "{{1/8+1/8}+{1/8+1/8}+{1/8+1/8}+{1/8+1/8}}");

        // getLevelList(0, True) is four quarters; (1, True) is eight eighths.
        let first: Vec<String> = bar
            .level_list(0, true)
            .iter()
            .map(MeterTerminal::to_string)
            .collect();
        assert_eq!(first, ["1/4", "1/4", "1/4", "1/4"]);
        let second: Vec<String> = bar
            .level_list(1, true)
            .iter()
            .map(MeterTerminal::to_string)
            .collect();
        assert_eq!(second, ["1/8"; 8]);

        // music21 reads this bar as two levels deep.
        assert_eq!(bar.depth(), 2);
        assert_eq!(bar.level_span(0).len(), 4);
        assert_eq!(bar.level_span(0)[1], (1.0, 2.0));

        // Every part of either level is the same ratio.
        assert!(bar.is_uniform_partition(0));
        assert!(bar.is_uniform_partition(1));
        // music21 reads the depth at an offset as 2, 1, 2 across the first beat.
        assert_eq!(bar.offset_to_depth(0.0, OffsetAlign::Quantize).unwrap(), 2);
        assert_eq!(bar.offset_to_depth(0.5, OffsetAlign::Quantize).unwrap(), 1);
        assert_eq!(bar.offset_to_depth(1.0, OffsetAlign::Quantize).unwrap(), 2);
    }

    #[test]
    fn a_bar_written_in_unequal_parts_is_not_uniform() {
        let mut bar = MeterTerminal::new(5, 8).unwrap();
        bar.partition_by_parts(&["2/8", "3/8"]).unwrap();
        assert!(!bar.is_uniform_partition(0));
        assert_eq!(bar.partition_display(), "2/8+3/8");
        assert_eq!(bar.depth(), 1);
    }

    #[test]
    fn subdividing_leaves_the_span_alone_and_returns_a_new_one() {
        let mut beat = MeterTerminal::new(1, 4).unwrap();
        beat.set_weight(0.5);
        let divided = beat.subdivide_by_count(2).unwrap();
        assert_eq!(divided.to_string(), "{1/8+1/8}");
        // music21's subdivide does not happen in place, and carries the
        // weight of the span it divided.
        assert!(beat.is_empty());
        assert!((divided.weight() - 0.5).abs() < 1e-9);

        let listed = MeterTerminal::new(5, 8)
            .unwrap()
            .subdivide_by_list(&[2, 3])
            .unwrap();
        assert_eq!(listed.to_string(), "{2/8+3/8}");
    }

    #[test]
    fn the_options_come_in_the_order_music21_offers_them() {
        use super::division_options_algorithmic;

        let offered: Vec<Vec<String>> = division_options_algorithmic(4, 4)
            .into_iter()
            .take(6)
            .collect();
        assert_eq!(
            offered,
            vec![
                vec!["1/4"; 4],
                vec!["1/8"; 8],
                vec!["1/16"; 16],
                vec!["1/2"; 2],
                vec!["4/4"],
                vec!["2/4"; 2],
            ]
        );

        // A compound meter is offered its threes first, and a five the two
        // groupings music21 keeps for it.
        assert_eq!(division_options_algorithmic(6, 8)[0], vec!["3/8"; 2]);
        assert_eq!(
            division_options_algorithmic(5, 8)[0],
            vec!["2/8".to_string(), "3/8".to_string()]
        );
    }

    /// music21's own examples, which is what this has to reproduce.
    #[test]
    fn a_bar_divides_the_way_music21_divides_it() {
        let mut four_four = MeterTerminal::from_ratio_string("4/4").unwrap();
        four_four.partition_by_count(2, true).unwrap();
        assert_eq!(four_four.partition_string(), "{1/2+1/2}");
        four_four.partition_by_count(4, true).unwrap();
        assert_eq!(four_four.partition_string(), "{1/4+1/4+1/4+1/4}");

        // Irregular meters take the grouping music21 keeps for them.
        let mut five_eight = MeterTerminal::from_ratio_string("5/8").unwrap();
        five_eight.partition_by_count(2, true).unwrap();
        assert_eq!(five_eight.partition_string(), "{2/8+3/8}");
        five_eight.partition_by_count(3, true).unwrap();
        assert_eq!(five_eight.partition_string(), "{2/8+2/8+1/8}");

        // A count nothing divides into falls back to the first option.
        let mut also_five = MeterTerminal::from_ratio_string("5/8").unwrap();
        also_five.partition_by_count(11, true).unwrap();
        assert_eq!(also_five.partition_string(), "{2/8+3/8}");
        assert!(
            MeterTerminal::from_ratio_string("5/8")
                .unwrap()
                .partition_by_count(11, false)
                .is_err()
        );

        // Compound meters divide into threes first.
        let mut six_eight = MeterTerminal::from_ratio_string("6/8").unwrap();
        six_eight.partition_by_count(2, true).unwrap();
        assert_eq!(six_eight.partition_string(), "{3/8+3/8}");
    }

    #[test]
    fn a_partition_must_come_to_what_the_bar_is() {
        let mut bar = MeterTerminal::from_ratio_string("4/4").unwrap();
        bar.partition_by_parts(&["3/4", "1/8", "1/8"]).unwrap();
        assert_eq!(bar.partition_string(), "{3/4+1/8+1/8}");
        assert!(bar.partition_by_parts(&["3/4", "1/8", "5/8"]).is_err());

        // A list of numerators is read against a finer note value where it
        // comes to a multiple of the meter's own.
        let mut halves = MeterTerminal::from_ratio_string("2/4").unwrap();
        halves.partition_by_list(&[1, 1]).unwrap();
        assert_eq!(halves.partition_string(), "{1/4+1/4}");
        halves.partition_by_list(&[1, 1, 1, 1]).unwrap();
        assert_eq!(halves.partition_string(), "{1/8+1/8+1/8+1/8}");
        assert!(halves.partition_by_list(&[1, 1, 1]).is_err());
    }

    #[test]
    fn an_offset_finds_the_part_it_falls_in() {
        let mut bar = MeterTerminal::from_ratio_string("4/4").unwrap();
        bar.partition_by_count(4, true).unwrap();
        assert_eq!(bar.offset_to_index(0.0).unwrap(), 0);
        assert_eq!(bar.offset_to_index(1.5).unwrap(), 1);
        assert_eq!(bar.offset_to_index(3.99).unwrap(), 3);
        assert_eq!(bar.offset_to_span(1.5, false).unwrap(), (1.0, 2.0));
        // Past the bar is an error, or the same offset read within it.
        assert!(bar.offset_to_index(4.0).is_err());
        assert!(bar.offset_to_index(-0.5).is_err());
        assert_eq!(bar.offset_to_span(5.5, true).unwrap(), (1.0, 2.0));
    }

    #[test]
    fn a_span_says_how_deep_and_how_flat_it_is() {
        let mut bar = MeterTerminal::from_ratio_string("4/4").unwrap();
        assert_eq!(bar.depth(), 0);
        bar.partition_by_count(2, true).unwrap();
        assert_eq!(bar.depth(), 1);
        bar.parts_mut()[0].partition_by_count(2, true).unwrap();
        assert_eq!(bar.depth(), 2);
        assert_eq!(bar.partition_string(), "{{1/4+1/4}+1/2}");
        assert_eq!(bar.flattened().len(), 3);
        assert!((bar.flattened()[0].quarter_length() - 1.0).abs() < 1e-9);
    }

    /// Read off music21's own `MeterSequence`.
    #[test]
    fn parts_joined_by_a_plus_are_read_as_a_partition() {
        let joined = MeterTerminal::from_partition_string("2/4+2/4").unwrap();
        assert_eq!(joined.to_string(), "{2/4+2/4}");
        assert_eq!((joined.numerator(), joined.denominator()), (4, 4));
        assert_eq!(joined.len(), 2);

        // A bare numerator takes the denominator of the part after it.
        let shared = MeterTerminal::from_partition_string("3+2/8").unwrap();
        assert_eq!(shared.to_string(), "{3/8+2/8}");
        assert_eq!((shared.numerator(), shared.denominator()), (5, 8));

        // Mixed denominators are written over the least they all divide into.
        let mixed = MeterTerminal::from_partition_string("3/4+2/4+1/8").unwrap();
        assert_eq!(mixed.to_string(), "{3/4+2/4+1/8}");
        assert_eq!((mixed.numerator(), mixed.denominator()), (11, 8));
        assert!((mixed.quarter_length() - 5.5).abs() < 1e-9);

        // One part joined to nothing is the span itself, undivided.
        let alone = MeterTerminal::from_partition_string("4/4").unwrap();
        assert_eq!(alone.to_string(), "4/4");
        assert!(alone.is_empty());

        assert!(MeterTerminal::from_partition_string("2+2").is_err());
        assert!(MeterTerminal::from_partition_string("half/4").is_err());
    }

    /// music21's own example: a bar of four quarters whose second quarter is
    /// divided into sixteenths.
    #[test]
    fn an_address_says_every_index_down_to_the_terminal() {
        let mut bar = MeterTerminal::new(4, 4).unwrap();
        bar.partition_by_count(4, true).unwrap();
        bar.parts_mut()[1].partition_by_count(4, true).unwrap();
        assert_eq!(bar.to_string(), "{1/4+{1/16+1/16+1/16+1/16}+1/4+1/4}");

        assert_eq!(bar.offset_to_address(0.0).unwrap(), [0]);
        assert_eq!(bar.offset_to_address(1.0).unwrap(), [1, 0]);
        assert_eq!(bar.offset_to_address(1.5).unwrap(), [1, 2]);
        assert_eq!(bar.offset_to_address(3.0).unwrap(), [3]);
        // How deep the terminal lies is how long its address is.
        assert_eq!(bar.offset_to_address(1.5).unwrap().len(), 2);
        assert!(bar.offset_to_address(-1.0).is_err());
    }

    /// Every expectation here was read off music21.
    #[test]
    fn a_span_flattens_to_what_its_leaves_come_to() {
        let mut bar = MeterTerminal::new(4, 4).unwrap();
        bar.partition_by_count(4, true).unwrap();
        bar.subdivide_partitions_equal(Some(2)).unwrap();

        let flat = bar.flatten().unwrap();
        assert_eq!(flat.to_string(), "{1/8+1/8+1/8+1/8+1/8+1/8+1/8+1/8}");
        // Written over what the leaves come to, not over the bar it came from.
        assert_eq!((flat.numerator(), flat.denominator()), (8, 8));
        assert_eq!(bar.flat_weights(), vec![0.125; 8]);
        assert_eq!(bar.level_weight(0), vec![0.25; 4]);
        assert_eq!(bar.level_weight(1), vec![0.125; 8]);
    }

    /// Every expectation here was read off music21.
    #[test]
    fn a_span_takes_the_division_and_the_shape_of_another() {
        let mut quarters = MeterTerminal::new(4, 4).unwrap();
        quarters.partition_by_count(4, true).unwrap();
        let mut halves = MeterTerminal::new(4, 4).unwrap();
        halves.partition_by_count(2, true).unwrap();

        quarters.partition_by_other(&halves).unwrap();
        assert_eq!(quarters.to_string(), "{1/2+1/2}");
        // The weight it had, which its new parts share out between them.
        assert_eq!(quarters.weight(), 1.0);

        // A division cannot relength the bar, whichever way it is asked for.
        let three = MeterTerminal::new(3, 4).unwrap();
        assert!(quarters.partition_by_other(&three).is_err());
        assert!(quarters.subdivide_by_other(&three).is_err());

        let thirds = MeterTerminal::from_partition_string("1/4+1/4+1/4").unwrap();
        let sixths = MeterTerminal::from_partition_string("3/8+3/8").unwrap();
        let divided = thirds.subdivide_by_other(&sixths).unwrap();
        // The other span becomes the one part, so its own parts lie a level
        // further down.
        assert_eq!(divided.to_string(), "{{3/8+3/8}}");
        assert_eq!(divided.weight(), 2.0);

        assert!(thirds.ratio_equal(&MeterTerminal::new(3, 4).unwrap()));
        assert!(!thirds.ratio_equal(&sixths));
    }
}