esoc-chart 0.1.0

// SPDX-License-Identifier: MIT OR Apache-2.0
//! Tick generation: "nice numbers" algorithm for readable axis ticks.

/// Computed tick marks for an axis.
#[derive(Clone, Debug)]
pub struct Ticks {
    /// Tick positions in data space.
    pub positions: Vec<f64>,
    /// Formatted tick labels.
    pub labels: Vec<String>,
}

/// Generate "nice" tick positions for a given data range.
///
/// Based on Paul Heckbert's "Nice Numbers for Graph Labels" algorithm.
/// Produces approximately `target_count` ticks at round intervals.
pub fn nice_ticks(min: f64, max: f64, target_count: usize) -> Ticks {
    if (max - min).abs() < 1e-15 {
        let label = format_tick(min);
        return Ticks {
            positions: vec![min],
            labels: vec![label],
        };
    }

    let target = target_count.max(2) as f64;
    let range = nice_num(max - min, false);
    let step = nice_num(range / (target - 1.0), true);

    let graph_min = (min / step).floor() * step;
    let graph_max = (max / step).ceil() * step;

    let mut positions = Vec::new();
    let mut v = graph_min;
    // Safety bound to prevent infinite loops
    let max_ticks = (target_count + 5) * 2;
    while v <= graph_max + step * 0.5 && positions.len() < max_ticks {
        positions.push(v);
        v += step;
    }

    let labels = positions.iter().map(|&v| format_tick(v)).collect();

    Ticks { positions, labels }
}

/// Generate nice tick positions for logarithmic axes.
pub fn nice_ticks_log(min: f64, max: f64) -> Ticks {
    let log_min = min.max(1e-15).log10().floor() as i32;
    let log_max = max.max(1e-15).log10().ceil() as i32;

    let mut positions = Vec::new();
    for exp in log_min..=log_max {
        positions.push(10.0_f64.powi(exp));
    }

    let labels = positions.iter().map(|&v| format_tick(v)).collect();
    Ticks { positions, labels }
}

/// Compute a "nice" number that is approximately equal to `x`.
///
/// If `round` is true, rounds to the nearest nice number.
/// If false, takes the ceiling.
fn nice_num(x: f64, round: bool) -> f64 {
    let exp = x.abs().log10().floor();
    let frac = x / 10.0_f64.powf(exp);

    let nice_frac = if round {
        if frac < 1.5 {
            1.0
        } else if frac < 3.0 {
            2.0
        } else if frac < 7.0 {
            5.0
        } else {
            10.0
        }
    } else if frac <= 1.0 {
        1.0
    } else if frac <= 2.0 {
        2.0
    } else if frac <= 5.0 {
        5.0
    } else {
        10.0
    };

    nice_frac * 10.0_f64.powf(exp)
}

/// Format a tick value as a concise label using SI prefixes and comma grouping.
pub fn format_tick(value: f64) -> String {
    if value == 0.0 {
        return "0".to_string();
    }

    let abs = value.abs();
    let sign = if value < 0.0 { "-" } else { "" };

    if abs >= 1e9 {
        let v = value / 1e9;
        format_si(v, sign, "B")
    } else if abs >= 1e6 {
        let v = value / 1e6;
        format_si(v, sign, "M")
    } else if abs >= 1e4 {
        // Comma-grouped integers
        format_with_commas(value)
    } else if abs >= 1.0 {
        if (value - value.round()).abs() < 1e-9 {
            format!("{}", value as i64)
        } else {
            format!("{value:.1}")
        }
    } else if abs >= 0.01 {
        format!("{value:.2}")
    } else if abs >= 1e-6 {
        // SI prefix for small numbers
        if abs >= 1e-3 {
            let v = value * 1e3;
            format_si(v, sign, "m")
        } else {
            let v = value * 1e6;
            format_si(v, sign, "\u{00B5}") // µ
        }
    } else {
        format!("{value:.2e}")
    }
}

fn format_si(v: f64, sign: &str, suffix: &str) -> String {
    let abs_v = v.abs();
    if (abs_v - abs_v.round()).abs() < 0.05 {
        format!("{sign}{}{suffix}", abs_v.round() as i64)
    } else {
        format!("{sign}{abs_v:.1}{suffix}")
    }
}

fn format_with_commas(value: f64) -> String {
    let rounded = value.round() as i64;
    let s = rounded.abs().to_string();
    let mut result = String::new();
    for (i, c) in s.chars().rev().enumerate() {
        if i > 0 && i % 3 == 0 {
            result.push(',');
        }
        result.push(c);
    }
    if rounded < 0 {
        result.push('-');
    }
    result.chars().rev().collect()
}

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

    #[test]
    fn test_nice_ticks_basic() {
        let ticks = nice_ticks(0.0, 100.0, 5);
        assert!(!ticks.positions.is_empty());
        assert!(ticks.positions[0] <= 0.0);
        assert!(*ticks.positions.last().unwrap() >= 100.0);
        // Steps should be round numbers
        if ticks.positions.len() >= 2 {
            let step = ticks.positions[1] - ticks.positions[0];
            assert!(step > 0.0);
        }
    }

    #[test]
    fn test_nice_ticks_small_range() {
        let ticks = nice_ticks(0.0, 1.0, 5);
        assert!(ticks.positions.len() >= 2);
    }

    #[test]
    fn test_format_tick() {
        assert_eq!(format_tick(0.0), "0");
        assert_eq!(format_tick(100.0), "100");
        assert_eq!(format_tick(2.5), "2.5");
        // SI prefixes
        assert_eq!(format_tick(1_000_000.0), "1M");
        assert_eq!(format_tick(2_500_000.0), "2.5M");
        assert_eq!(format_tick(1_000_000_000.0), "1B");
        assert_eq!(format_tick(-3_000_000.0), "-3M");
        // Comma grouping
        assert_eq!(format_tick(12_000.0), "12,000");
        assert_eq!(format_tick(100_000.0), "100,000");
        // Small numbers
        assert_eq!(format_tick(0.001), "1m");
        assert_eq!(format_tick(0.0002), "200\u{00B5}");
    }

    #[test]
    fn test_nice_ticks_same_value() {
        let ticks = nice_ticks(5.0, 5.0, 5);
        assert_eq!(ticks.positions.len(), 1);
    }
}