町内会・老人会・学校PTAでそのまま配れる「地域サイバー防犯かわら版」：冷蔵庫に貼れる安心チェックシート

#![forbid(unsafe_code)]

/// Computes relative luminance L in [0.0, 1.0] per WCAG 2.x specification from sRGB (0..=255).
#[inline]
pub fn relative_luminance(r: u8, g: u8, b: u8) -> f64 {
    fn channel_linear(c: u8) -> f64 {
        let s = (c as f64) / 255.0;
        if s <= 0.04045 {
            s / 12.92
        } else {
            ((s + 0.055) / 1.055).powf(2.4)
        }
    }
    0.2126 * channel_linear(r) + 0.7152 * channel_linear(g) + 0.0722 * channel_linear(b)
}

/// Computes WCAG contrast ratio (1.0 to 21.0) between foreground and background RGB colors.
#[inline]
pub fn contrast_ratio(fg: (u8, u8, u8), bg: (u8, u8, u8)) -> f64 {
    let l1 = relative_luminance(fg.0, fg.1, fg.2);
    let l2 = relative_luminance(bg.0, bg.1, bg.2);
    let (bright, dark) = if l1 > l2 { (l1, l2) } else { (l2, l1) };
    (bright + 0.05) / (dark + 0.05)
}

/// Computes Hick-Hyman reaction time in milliseconds: RT = a + b * log2(n + 1).
#[inline]
pub fn hick_hyman_reaction_ms(num_choices: usize, base_ms: f64, slope_ms_per_bit: f64) -> f64 {
    if num_choices == 0 {
        return 0.0;
    }
    let entropy_bits = ((num_choices + 1) as f64).log2();
    base_ms + slope_ms_per_bit * entropy_bits
}

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn verify_print_ergonomics() {
        let ratio = contrast_ratio((20, 20, 19), (250, 248, 245));
        assert!(ratio >= 14.0); — Exceeds WCAG AAA (7.0:1) by >2x
        let rt_3_choices = hick_hyman_reaction_ms(3, 250.0, 155.0);
        assert!(rt_3_choices <= 600.0);
    }
}