quadraui 0.1.0

//! Shared `U+2800`-block braille dot packing, used by every TUI
//! rasteriser that needs sub-cell resolution: [`super::chart`]'s line
//! charts and [`super::minimap`]'s density view (#382).
//!
//! Lifted out of `tui/chart.rs` rather than duplicated: getting the bit
//! order wrong transposes the whole image in a way that still looks
//! plausible, so there must be exactly one copy of it (#382 review note).

/// Braille dot offsets: `(row_in_cell, col_in_cell) -> bit index`.
/// Standard Unicode braille ordering — a terminal cell is 2 dots wide
/// (`col_in_cell` in `0..2`) by 4 dots tall (`row_in_cell` in `0..4`).
pub(crate) const BRAILLE_OFFSETS: [(usize, usize); 8] = [
    (0, 0), // bit 0
    (1, 0), // bit 1
    (2, 0), // bit 2
    (0, 1), // bit 3
    (1, 1), // bit 4
    (2, 1), // bit 5
    (3, 0), // bit 6
    (3, 1), // bit 7
];

/// Pack one 2x4 dot cell into its `U+2800`-block braille codepoint.
/// `dot_at(row, col)` is queried for every one of the 8 dots
/// (`row` in `0..4`, `col` in `0..2`) and should return whether that dot
/// is set. An all-`false` cell packs to `U+2800` itself (blank braille),
/// not a space — callers that want to skip painting an empty cell decide
/// that themselves by comparing the result to `'\u{2800}'`.
pub(crate) fn pack_braille_cell(mut dot_at: impl FnMut(usize, usize) -> bool) -> char {
    let mut code: u32 = 0x2800;
    for (bit, &(row, col)) in BRAILLE_OFFSETS.iter().enumerate() {
        if dot_at(row, col) {
            code |= 1 << bit;
        }
    }
    char::from_u32(code).unwrap_or(' ')
}

/// Threshold a dot's coverage (`covered` non-whitespace source columns out
/// of `bucket_width` total) against a 4x4 ordered-dither (Bayer) matrix,
/// indexed by the dot's own `(row, col)` position within the whole
/// rendered grid (not just within its cell) so that the dither pattern
/// tiles consistently across the entire minimap strip rather than
/// repeating identically inside every cell.
///
/// [`super::minimap`]'s density view decides "is this dot set?" with a
/// boolean OR over the dot's source-column bucket (`any(|c|
/// !c.is_whitespace())`), which saturates every dot from the end of a
/// line's indent onward as soon as the bucket widens past one column —
/// every row of real code runs to the strip's right edge with no
/// line-length signal at all. An ordered dither spreads that "is there
/// *any* code here" boolean across many dots' worth of threshold instead:
/// a sparsely-covered bucket only lights up the small subset of dot
/// positions whose threshold value happens to be low, while a
/// fully-covered bucket lights up every position — which is what produces
/// a raggedy, VS-Code-like right edge instead of a solid wall.
///
/// Pure integer arithmetic — one multiply and one compare, no floating
/// point, no lookahead, no per-call allocation. A fully-covered bucket
/// (`covered == bucket_width`) always returns `true` and an
/// all-whitespace bucket (`covered == 0`) always returns `false` —
/// dithering only has any effect strictly *between* those two extremes.
///
/// Re-exported from [`crate::primitives::minimap`] rather than defined
/// here: that module's `sample_blocks` uses the exact same matrix and
/// threshold formula to make an analogous decision one granularity
/// coarser — whether a *block*'s per-column coverage (several real
/// buffer lines folded into one output row) reads back non-blank. A
/// single shared definition makes both compositions provably the same
/// dither policy applied twice, not two that happen to agree today.
pub(crate) use crate::primitives::minimap::dither_threshold_met;

/// Set every dot on the straight line between `(x0, y0)` and `(x1, y1)`
/// in `grid` (indexed `grid[row][col]`, i.e. `grid[y][x]`), including
/// both endpoints. Shared by every rasteriser in this crate that needs to
/// connect two dot-grid coordinates with a solid stroke — [`super::chart`]'s
/// line charts and [`super::canvas`]'s `DrawOp::Line`/`DrawOp::Path`
/// degrade both call this rather than keeping their own,
/// independently-tuned copy of a line-rasterisation loop, which is
/// exactly the kind of drift this module's own doc comment already warns
/// about for the bit-packing table above.
///
/// Out-of-bounds coordinates (`iy >= grid.len()` or `ix >= grid[0].len()`)
/// are silently skipped rather than panicking — a caller that clamps its
/// own endpoints to the grid never hits this, but the interpolated path
/// between two in-bounds endpoints cannot itself leave the grid (each
/// step is a convex combination of two in-bounds points), so this guard
/// is defensive rather than load-bearing.
///
/// **`(x0, y0) == (x1, y1)` is a no-op** — every caller already sets a
/// coincident single point's own dot itself before calling this for the
/// *connecting* segment, so a caller that wants a lone point rendered
/// when both endpoints coincide must set it separately; see
/// [`super::canvas`]'s `DrawOp::Line` handling for an example.
pub(crate) fn interpolate_dots(grid: &mut [Vec<bool>], x0: usize, y0: usize, x1: usize, y1: usize) {
    let dx = (x1 as isize - x0 as isize).abs();
    let dy = (y1 as isize - y0 as isize).abs();
    let steps = dx.max(dy);
    if steps == 0 {
        return;
    }
    for step in 0..=steps {
        let t = step as f64 / steps as f64;
        let ix = (x0 as f64 + t * (x1 as f64 - x0 as f64)).round() as usize;
        let iy = (y0 as f64 + t * (y1 as f64 - y0 as f64)).round() as usize;
        if iy < grid.len() && ix < grid[0].len() {
            grid[iy][ix] = true;
        }
    }
}

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

    #[test]
    fn all_dots_off_packs_to_blank_braille() {
        assert_eq!(pack_braille_cell(|_, _| false), '\u{2800}');
    }

    #[test]
    fn all_dots_on_packs_to_full_braille_block() {
        assert_eq!(pack_braille_cell(|_, _| true), '\u{28FF}');
    }

    #[test]
    fn single_dot_sets_the_documented_bit() {
        // bit 0 -> (row 0, col 0)
        assert_eq!(pack_braille_cell(|r, c| (r, c) == (0, 0)), '\u{2801}');
        // bit 3 -> (row 0, col 1)
        assert_eq!(pack_braille_cell(|r, c| (r, c) == (0, 1)), '\u{2808}');
        // bit 6 -> (row 3, col 0)
        assert_eq!(pack_braille_cell(|r, c| (r, c) == (3, 0)), '\u{2840}');
        // bit 7 -> (row 3, col 1)
        assert_eq!(pack_braille_cell(|r, c| (r, c) == (3, 1)), '\u{2880}');
    }

    // ── dither_threshold_met (issue #1007) ─────────────────────────────

    #[test]
    fn zero_coverage_never_meets_the_threshold() {
        for row in 0..4 {
            for col in 0..4 {
                assert!(!dither_threshold_met(0, 6, row, col));
            }
        }
    }

    #[test]
    fn zero_width_bucket_never_meets_the_threshold() {
        assert!(!dither_threshold_met(0, 0, 0, 0));
        assert!(!dither_threshold_met(5, 0, 0, 0));
    }

    #[test]
    fn full_coverage_always_meets_the_threshold() {
        // A fully-covered bucket must light up at every dot position,
        // including the matrix's own largest threshold (15) -- otherwise
        // a solid run of non-whitespace would still leave holes.
        for row in 0..4 {
            for col in 0..4 {
                assert!(
                    dither_threshold_met(6, 6, row, col),
                    "full coverage must always set the dot at ({row}, {col})"
                );
            }
        }
    }

    #[test]
    fn partial_coverage_sets_only_some_dot_positions() {
        // 1-of-6 covered: only the positions whose Bayer threshold is low
        // enough should light up, and it must not be all-or-nothing across
        // the matrix -- that's the whole point of dithering a partially
        // covered bucket instead of booleans-OR-ing it.
        let set_count = (0..4)
            .flat_map(|row| (0..4).map(move |col| (row, col)))
            .filter(|&(row, col)| dither_threshold_met(1, 6, row, col))
            .count();
        assert!(
            set_count > 0 && set_count < 16,
            "expected a strict subset of the 16 dot positions to light up, got {set_count}/16"
        );
    }

    #[test]
    fn threshold_is_indexed_by_absolute_position_not_just_local_position() {
        // Position (0, 0) and (4, 4) share the same `& 3` fold, so they
        // must resolve identically -- confirms the matrix tiles rather
        // than being looked up some other way.
        assert_eq!(
            dither_threshold_met(3, 6, 0, 0),
            dither_threshold_met(3, 6, 4, 4)
        );
    }
}