ballistics_engine/

bc_estimation.rs

use crate::BCSegmentData;

/// Resolve a velocity-keyed BC table without assuming segment order.
///
/// Bands are half-open (`velocity_min <= v < velocity_max`), so a shared boundary belongs to
/// the upper band. Below/above global coverage, clamp to the lowest/highest-velocity band;
/// an interior coverage gap or empty table uses the caller's projectile-specific scalar BC.
pub(crate) fn velocity_segment_bc(
    velocity_fps: f64,
    segments: &[BCSegmentData],
    fallback_bc: f64,
) -> f64 {
    if let Some(segment) = segments.iter().find(|segment| {
        velocity_fps >= segment.velocity_min && velocity_fps < segment.velocity_max
    }) {
        return segment.bc_value;
    }

    let lowest = segments
        .iter()
        .min_by(|a, b| a.velocity_min.total_cmp(&b.velocity_min));
    if let Some(segment) = lowest {
        if velocity_fps < segment.velocity_min {
            return segment.bc_value;
        }
    }

    let highest = segments
        .iter()
        .max_by(|a, b| a.velocity_max.total_cmp(&b.velocity_max));
    if let Some(segment) = highest {
        if velocity_fps >= segment.velocity_max {
            return segment.bc_value;
        }
    }

    fallback_bc
}

/// Bullet type classification based on model name
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum BulletType {
    MatchBoatTail,
    MatchFlatBase,
    HuntingBoatTail,
    HuntingFlatBase,
    VldHighBc,
    Hybrid,
    FMJ,
    RoundNose,
    Unknown,
}

/// BC degradation factors for different bullet types
pub struct BulletTypeFactors {
    pub drop: f64,
    pub transition_curve: f64,
}

impl BulletType {
    /// Get degradation factors for this bullet type
    pub fn get_factors(&self) -> BulletTypeFactors {
        match self {
            BulletType::MatchBoatTail => BulletTypeFactors {
                drop: 0.075, // 7.5% total drop for match boat tail
                transition_curve: 0.3,
            },
            BulletType::MatchFlatBase => BulletTypeFactors {
                drop: 0.10, // 10% for match flat base
                transition_curve: 0.35,
            },
            BulletType::HuntingBoatTail => BulletTypeFactors {
                drop: 0.15, // 15% for hunting boat tail
                transition_curve: 0.45,
            },
            BulletType::HuntingFlatBase => BulletTypeFactors {
                drop: 0.20, // 20% for hunting flat base
                transition_curve: 0.5,
            },
            BulletType::VldHighBc => BulletTypeFactors {
                drop: 0.05, // 5% for VLD (very low drag)
                transition_curve: 0.25,
            },
            BulletType::Hybrid => BulletTypeFactors {
                drop: 0.06, // 6% for hybrid designs
                transition_curve: 0.28,
            },
            BulletType::FMJ => BulletTypeFactors {
                drop: 0.12, // 12% for military ball
                transition_curve: 0.4,
            },
            BulletType::RoundNose => BulletTypeFactors {
                drop: 0.35, // 35% for round nose
                transition_curve: 0.7,
            },
            BulletType::Unknown => BulletTypeFactors {
                drop: 0.15, // Conservative 15%
                transition_curve: 0.5,
            },
        }
    }
}

/// BC segment estimator based on physics and known patterns
pub struct BCSegmentEstimator;

impl BCSegmentEstimator {
    /// Identify bullet type from model name and characteristics.
    ///
    /// This compatibility entry point interprets `bc_value` as a G1 BC. Call
    /// [`Self::identify_bullet_type_for_drag_model`] when the reference drag
    /// model is known.
    pub fn identify_bullet_type(
        model: &str,
        weight: f64,
        caliber: f64,
        bc_value: Option<f64>,
    ) -> BulletType {
        Self::identify_bullet_type_for_drag_model(model, weight, caliber, bc_value, "G1")
    }

    /// Identify bullet type while interpreting the BC in its reference-model space.
    pub fn identify_bullet_type_for_drag_model(
        model: &str,
        weight: f64,
        caliber: f64,
        bc_value: Option<f64>,
        drag_model: &str,
    ) -> BulletType {
        let model_lower = model.to_lowercase();

        // VLD/High BC bullets
        if model_lower.contains("vld")
            || model_lower.contains("berger")
            || model_lower.contains("hybrid")
            || model_lower.contains("elite")
        {
            if model_lower.contains("hybrid") {
                return BulletType::Hybrid;
            }
            return BulletType::VldHighBc;
        }

        // Match bullets (competition/target)
        if model_lower.contains("smk")
            || model_lower.contains("matchking")
            || model_lower.contains("match")
            || model_lower.contains("bthp")
            || model_lower.contains("competition")
            || model_lower.contains("target")
            || model_lower.contains("a-max")
            || model_lower.contains("eld-m")
            || model_lower.contains("scenar")
            || model_lower.contains("x-ring")
        {
            // Check for boat tail
            if model_lower.contains("bt") || model_lower.contains("boat") {
                return BulletType::MatchBoatTail;
            }
            // Check if high BC indicates boat tail (guard sd>0: calculate_sectional_density
            // returns 0 for non-positive caliber, which would make bc/sd == +Inf).
            if let Some(bc) = bc_value {
                let sd = Self::calculate_sectional_density(weight, caliber);
                if sd > 0.0 && Self::classification_bc_sd_ratio(bc, sd, drag_model) > 1.6 {
                    return BulletType::MatchBoatTail;
                }
            }
            return BulletType::MatchFlatBase;
        }

        // Hunting bullets (expanding)
        if model_lower.contains("gameking")
            || model_lower.contains("hunting")
            || model_lower.contains("sst")
            || model_lower.contains("eld-x")
            || model_lower.contains("partition")
            || model_lower.contains("accubond")
            || model_lower.contains("core-lokt")
            || model_lower.contains("ballistic tip")
            || model_lower.contains("v-max")
            || model_lower.contains("hornady sp")
            || model_lower.contains("interlock")
            || model_lower.contains("tsx")
        {
            // Check for boat tail
            if model_lower.contains("bt")
                || model_lower.contains("boat")
                || model_lower.contains("sst")
                || model_lower.contains("accubond")
            {
                return BulletType::HuntingBoatTail;
            }
            return BulletType::HuntingFlatBase;
        }

        // FMJ/Military
        if model_lower.contains("fmj")
            || model_lower.contains("ball")
            || model_lower.contains("m80")
            || model_lower.contains("m855")
            || model_lower.contains("tracer")
        {
            return BulletType::FMJ;
        }

        // Round nose
        if model_lower.contains("rn")
            || model_lower.contains("round nose")
            || model_lower.contains("rnsp")
        {
            return BulletType::RoundNose;
        }

        // Use BC value as hint if available. Guard sd>0 (zero for non-positive caliber)
        // so a degenerate input falls through to Unknown instead of dividing by zero
        // (bc/0 == +Inf, which would silently classify as VldHighBc).
        if let Some(bc) = bc_value {
            let sd = Self::calculate_sectional_density(weight, caliber);
            if sd > 0.0 {
                let bc_to_sd_ratio = Self::classification_bc_sd_ratio(bc, sd, drag_model);

                if bc_to_sd_ratio > 1.8 {
                    return BulletType::VldHighBc;
                } else if bc_to_sd_ratio > 1.5 {
                    return BulletType::MatchBoatTail;
                } else if bc_to_sd_ratio < 1.2 {
                    return BulletType::HuntingFlatBase;
                }
            }
        }

        BulletType::Unknown
    }

    /// Convert the reference-model-dependent BC/SD ratio into the G1 space used
    /// by the legacy coarse classification thresholds above. Typical boat-tail
    /// G1 BCs are approximately twice their G7 BCs; this normalization prevents
    /// ordinary G7 match bullets from looking like low-BC G1 flat-base bullets.
    fn classification_bc_sd_ratio(bc: f64, sd: f64, drag_model: &str) -> f64 {
        let g1_equivalent_bc = if drag_model.eq_ignore_ascii_case("G7") {
            bc * 2.0
        } else {
            bc
        };
        g1_equivalent_bc / sd
    }

    /// Calculate sectional density (SD) from weight and caliber
    pub fn calculate_sectional_density(weight_grains: f64, caliber_inches: f64) -> f64 {
        // SD = weight / (7000 * caliber^2)
        // Protect against division by zero or negative caliber
        if caliber_inches <= 0.0 {
            return 0.0;
        }
        weight_grains / (7000.0 * caliber_inches * caliber_inches)
    }

    /// Estimate BC segments based on bullet characteristics
    pub fn estimate_bc_segments(
        base_bc: f64,
        caliber: f64,
        weight: f64,
        model: &str,
        drag_model: &str,
    ) -> Vec<BCSegmentData> {
        // Identify bullet type
        let bullet_type = Self::identify_bullet_type_for_drag_model(
            model,
            weight,
            caliber,
            Some(base_bc),
            drag_model,
        );
        let type_factors = bullet_type.get_factors();

        // Calculate sectional density
        let sd = Self::calculate_sectional_density(weight, caliber);

        // Adjust BC drop based on sectional density
        // Higher SD = more stable BC
        let sd_factor = (sd / 0.25).max(0.7).min(1.3);
        let nominal_drop = type_factors.drop;

        // Generate segments based on bullet type
        let mut segments = Vec::new();

        // Determine velocity ranges and BC retention factors
        match bullet_type {
            BulletType::MatchBoatTail => {
                // Match boat tail - minimal BC degradation
                segments.push(BCSegmentData {
                    velocity_min: 2800.0,
                    velocity_max: 5000.0,
                    bc_value: base_bc * 1.000,
                });
                segments.push(BCSegmentData {
                    velocity_min: 2400.0,
                    velocity_max: 2800.0,
                    bc_value: base_bc * 0.985,
                });
                segments.push(BCSegmentData {
                    velocity_min: 2000.0,
                    velocity_max: 2400.0,
                    bc_value: base_bc * 0.965,
                });
                segments.push(BCSegmentData {
                    velocity_min: 1600.0,
                    velocity_max: 2000.0,
                    bc_value: base_bc * 0.945,
                });
                segments.push(BCSegmentData {
                    velocity_min: 0.0,
                    velocity_max: 1600.0,
                    bc_value: base_bc * 0.925,
                });
            }
            BulletType::VldHighBc | BulletType::Hybrid => {
                // VLD/Hybrid - very stable BC
                segments.push(BCSegmentData {
                    velocity_min: 2800.0,
                    velocity_max: 5000.0,
                    bc_value: base_bc * 1.000,
                });
                segments.push(BCSegmentData {
                    velocity_min: 2200.0,
                    velocity_max: 2800.0,
                    bc_value: base_bc * 0.990,
                });
                segments.push(BCSegmentData {
                    velocity_min: 1600.0,
                    velocity_max: 2200.0,
                    bc_value: base_bc * 0.970,
                });
                segments.push(BCSegmentData {
                    velocity_min: 0.0,
                    velocity_max: 1600.0,
                    bc_value: base_bc * 0.950,
                });
            }
            BulletType::HuntingBoatTail => {
                // Hunting boat tail - moderate degradation
                segments.push(BCSegmentData {
                    velocity_min: 2600.0,
                    velocity_max: 5000.0,
                    bc_value: base_bc * 1.000,
                });
                segments.push(BCSegmentData {
                    velocity_min: 2200.0,
                    velocity_max: 2600.0,
                    bc_value: base_bc * 0.960,
                });
                segments.push(BCSegmentData {
                    velocity_min: 1800.0,
                    velocity_max: 2200.0,
                    bc_value: base_bc * 0.900,
                });
                segments.push(BCSegmentData {
                    velocity_min: 0.0,
                    velocity_max: 1800.0,
                    bc_value: base_bc * 0.850,
                });
            }
            _ => {
                // Default degradation profile
                segments.push(BCSegmentData {
                    velocity_min: 2800.0,
                    velocity_max: 5000.0,
                    bc_value: base_bc,
                });

                let transonic_bc = base_bc * (1.0 - nominal_drop * 0.3);
                segments.push(BCSegmentData {
                    velocity_min: 1800.0,
                    velocity_max: 2800.0,
                    bc_value: transonic_bc,
                });

                let subsonic_bc = base_bc * (1.0 - nominal_drop);
                segments.push(BCSegmentData {
                    velocity_min: 0.0,
                    velocity_max: 1800.0,
                    bc_value: subsonic_bc,
                });
            }
        }

        // G7 reference drag follows modern boat-tail projectiles more closely,
        // so their banded BC varies less than the G1-shaped ladders above. Scale
        // each loss from nominal rather than the BC itself: this leaves the muzzle
        // band unchanged by this adjustment and makes the model effective for every
        // named and default profile. Do not run the identity algebra for G1, so
        // its established floating-point outputs remain bit-for-bit unchanged.
        if drag_model.eq_ignore_ascii_case("G7") {
            const G7_DROP_SCALE: f64 = 0.8;
            for segment in &mut segments {
                let drop_from_nominal = base_bc - segment.bc_value;
                segment.bc_value = base_bc - drop_from_nominal * G7_DROP_SCALE;
            }
        }

        // Sectional density shapes only the degradation depth. Scaling the BC
        // itself would alter the user's published muzzle value and would apply SD
        // twice in the default profile. Skip identity algebra so SD=0.25 keeps
        // established output bits unchanged.
        if sd_factor != 1.0 {
            for segment in &mut segments {
                let drop_from_nominal = base_bc - segment.bc_value;
                segment.bc_value = base_bc - drop_from_nominal / sd_factor;
            }
        }

        segments
    }
}

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

    #[test]
    fn test_bullet_type_identification() {
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type("168gr SMK", 168.0, 0.308, None),
            BulletType::MatchFlatBase
        );
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type("168gr SMK BT", 168.0, 0.308, None),
            BulletType::MatchBoatTail
        );
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type("150gr SST", 150.0, 0.308, None),
            BulletType::HuntingBoatTail
        );
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type("147gr FMJ", 147.0, 0.308, None),
            BulletType::FMJ
        );
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type("180gr RN", 180.0, 0.308, None),
            BulletType::RoundNose
        );
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type("168gr VLD", 168.0, 0.308, None),
            BulletType::VldHighBc
        );
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type("Some bullet", 150.0, 0.308, None),
            BulletType::Unknown
        );
    }

    #[test]
    fn test_sectional_density() {
        let sd = BCSegmentEstimator::calculate_sectional_density(168.0, 0.308);
        assert!((sd - 0.253).abs() < 0.001);
    }

    #[test]
    fn test_bc_estimation() {
        let segments =
            BCSegmentEstimator::estimate_bc_segments(0.450, 0.308, 168.0, "168gr SMK", "G1");

        // Match rifles typically have 4 segments
        assert!(segments.len() >= 3);
        // First segment should be close to base BC
        assert!((segments[0].bc_value - 0.450).abs() < 0.05);
        // BC should degrade at lower velocities
        assert!(segments[segments.len() - 1].bc_value < segments[0].bc_value);
    }

    #[test]
    fn g7_transition_adjustment_softens_each_band_drop() {
        let base_bc = 0.5;
        // SD = 0.25 exactly, so the independent sectional-density adjustment is neutral.
        let caliber = 1.0;
        let weight = 1750.0;

        for model in ["SMK BT", "FMJ"] {
            let g1 =
                BCSegmentEstimator::estimate_bc_segments(base_bc, caliber, weight, model, "G1");
            let g7 =
                BCSegmentEstimator::estimate_bc_segments(base_bc, caliber, weight, model, "G7");
            let lowercase_g7 =
                BCSegmentEstimator::estimate_bc_segments(base_bc, caliber, weight, model, "g7");
            assert_eq!(g7.len(), g1.len());
            assert_eq!(lowercase_g7.len(), g7.len());

            for ((g1_band, g7_band), lowercase_band) in
                g1.iter().zip(&g7).zip(&lowercase_g7)
            {
                assert_eq!(g7_band.velocity_min, g1_band.velocity_min);
                assert_eq!(g7_band.velocity_max, g1_band.velocity_max);
                assert_eq!(lowercase_band.velocity_min, g7_band.velocity_min);
                assert_eq!(lowercase_band.velocity_max, g7_band.velocity_max);
                assert_eq!(lowercase_band.bc_value.to_bits(), g7_band.bc_value.to_bits());
                let expected_g7 = base_bc - (base_bc - g1_band.bc_value) * 0.8;
                assert!(
                    (g7_band.bc_value - expected_g7).abs() < 1e-12,
                    "{model} band {}-{} did not soften the G1 loss: G1={}, G7={}, expected={expected_g7}",
                    g1_band.velocity_min,
                    g1_band.velocity_max,
                    g1_band.bc_value,
                    g7_band.bc_value
                );
            }
        }
    }

    #[test]
    fn sectional_density_shapes_only_band_degradation() {
        let base_bc = 0.5;
        let caliber = 0.224;
        let weight = 77.0;
        let sd = BCSegmentEstimator::calculate_sectional_density(weight, caliber);
        let sd_factor = (sd / 0.25).clamp(0.7, 1.3);

        for drag_model in ["G1", "G7"] {
            let model_drop_scale = if drag_model == "G7" { 0.8 } else { 1.0 };
            for (model, raw_retentions) in [
                ("SMK BT", &[1.0, 0.985, 0.965, 0.945, 0.925][..]),
                ("FMJ", &[1.0, 0.964, 0.88][..]),
            ] {
                let segments = BCSegmentEstimator::estimate_bc_segments(
                    base_bc,
                    caliber,
                    weight,
                    model,
                    drag_model,
                );
                assert_eq!(segments.len(), raw_retentions.len());

                for (segment, raw_retention) in segments.iter().zip(raw_retentions) {
                    let raw_drop = base_bc * (1.0 - raw_retention) * model_drop_scale;
                    let expected = base_bc - raw_drop / sd_factor;
                    assert!(
                        (segment.bc_value - expected).abs() < 1e-12,
                        "{drag_model} {model} band {}-{} misapplied SD: got {}, expected {expected}",
                        segment.velocity_min,
                        segment.velocity_max,
                        segment.bc_value
                    );
                }
                assert_eq!(
                    segments[0].bc_value.to_bits(),
                    base_bc.to_bits(),
                    "published muzzle BC must remain exact for {drag_model} {model}"
                );
            }
        }

        // High SD used to multiply every band above nominal and then cap them
        // all to base_bc, erasing the degradation ladder.
        let high_sd_base_bc = 0.3;
        let high_sd_segments = BCSegmentEstimator::estimate_bc_segments(
            high_sd_base_bc,
            0.308,
            220.0,
            "SMK BT",
            "G7",
        );
        assert_eq!(high_sd_segments[0].bc_value.to_bits(), high_sd_base_bc.to_bits());
        assert!(high_sd_segments.last().unwrap().bc_value < high_sd_base_bc);
        assert!((high_sd_segments.last().unwrap().bc_value - 0.28615384615384615).abs() < 1e-12);
    }

    #[test]
    fn generic_g7_bc_uses_g7_classification_space() {
        // A representative 175 gr .308 match bullet. Its G7 BC is ordinary for a
        // boat-tail projectile, but the same numeric value looks like a blunt
        // flat-base bullet when interpreted with the G1 BC/SD thresholds.
        let base_bc = 0.243;
        let segments = BCSegmentEstimator::estimate_bc_segments(base_bc, 0.308, 175.0, "", "G7");

        assert!(
            segments.len() >= 4,
            "G7 match bullet should use a near-flat match/VLD ladder: {segments:?}"
        );
        let subsonic_bc = segments.last().unwrap().bc_value;
        assert!(
            subsonic_bc >= base_bc * 0.92,
            "G7 match bullet was over-degraded: {base_bc} -> {subsonic_bc}"
        );

        // The normalization is deliberately equivalent to classifying the
        // approximate G1 BC, while the G7 transition adjustment then softens
        // the loss within that same ladder and the legacy entry point remains
        // G1-compatible.
        let g1_segments =
            BCSegmentEstimator::estimate_bc_segments(base_bc * 2.0, 0.308, 175.0, "", "G1");
        assert_eq!(segments.len(), g1_segments.len());
        let mut saw_g7_softening = false;
        for (g7, g1) in segments.iter().zip(&g1_segments) {
            assert_eq!(g7.velocity_min.to_bits(), g1.velocity_min.to_bits());
            assert_eq!(g7.velocity_max.to_bits(), g1.velocity_max.to_bits());
            let g7_retention = g7.bc_value / base_bc;
            let g1_retention = g1.bc_value / (base_bc * 2.0);
            assert!(g7_retention + 1e-12 >= g1_retention);
            saw_g7_softening |= g7_retention > g1_retention + 1e-12;
        }
        assert!(saw_g7_softening);

        let legacy_g1 = BCSegmentEstimator::identify_bullet_type("", 175.0, 0.308, Some(base_bc));
        assert_eq!(legacy_g1, BulletType::HuntingFlatBase);
        assert_eq!(
            legacy_g1,
            BCSegmentEstimator::identify_bullet_type_for_drag_model(
                "",
                175.0,
                0.308,
                Some(base_bc),
                "G1",
            )
        );
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type_for_drag_model(
                "175gr SMK",
                175.0,
                0.308,
                Some(base_bc),
                "g7",
            ),
            BulletType::MatchBoatTail
        );
        assert_eq!(
            BCSegmentEstimator::identify_bullet_type_for_drag_model(
                "",
                175.0,
                0.0,
                Some(base_bc),
                "G7",
            ),
            BulletType::Unknown
        );
    }
}