sc_neurocore_engine/synapses/

mod.rs

// SPDX-License-Identifier: AGPL-3.0-or-later | Commercial license available
// © Concepts 1996–2026 Miroslav Šotek. All rights reserved.
// © Code 2020–2026 Miroslav Šotek. All rights reserved.
// ORCID: 0009-0009-3560-0851
// Contact: www.anulum.li | protoscience@anulum.li
// SC-NeuroCore — Stochastic STDP Synapse

//! # Stochastic STDP Synapse
//!
//! Implements Spike-Timing-Dependent Plasticity (STDP) for stochastic bitstreams.
//! Optimized for on-chip adaptation.

use crate::neuron::mask;

/// Parameters for the STDP rule.
#[derive(Clone, Copy, Debug)]
pub struct StdpParams {
    pub a_plus: i16,
    pub a_minus: i16,
    pub decay: i16,
    pub w_min: i16,
    pub w_max: i16,
}

/// A synapse with STDP learning capability.
#[derive(Clone, Debug)]
pub struct StdpSynapse {
    /// Current weight value (fixed-point).
    pub weight: i16,
    /// Pre-synaptic trace.
    pub trace_pre: i16,
    /// Post-synaptic trace.
    pub trace_post: i16,
    /// Data width.
    pub data_width: u32,
    /// Fraction bits.
    pub fraction: u32,
}

impl StdpSynapse {
    pub fn new(initial_weight: i16, data_width: u32, fraction: u32) -> Self {
        Self {
            weight: initial_weight,
            trace_pre: 0,
            trace_post: 0,
            data_width,
            fraction,
        }
    }

    /// Update traces and weight based on pre and post spikes.
    ///
    /// This is a simplified, hardware-friendly STDP rule:
    ///   - On pre-spike: trace_pre += A_plus; weight += trace_post * rate
    ///   - On post-spike: trace_post += A_minus; weight += trace_pre * rate
    ///   - Traces decay over time.
    pub fn step(&mut self, pre_spike: bool, post_spike: bool, params: &StdpParams) {
        // 1. Decay traces
        self.trace_pre = mask(
            (self.trace_pre as i32 * params.decay as i32) >> self.fraction,
            self.data_width,
        );
        self.trace_post = mask(
            (self.trace_post as i32 * params.decay as i32) >> self.fraction,
            self.data_width,
        );

        // 2. Update traces
        if pre_spike {
            self.trace_pre = mask(
                self.trace_pre as i32 + params.a_plus as i32,
                self.data_width,
            );
        }
        if post_spike {
            self.trace_post = mask(
                self.trace_post as i32 + params.a_minus as i32,
                self.data_width,
            );
        }

        // 3. Update weight — mutually exclusive per timestep.
        // Simultaneous spikes → LTP (pre-before-post convention).
        if post_spike {
            let dw = (self.trace_pre as i32 * params.a_plus.abs() as i32) >> self.fraction;
            let new_w = (self.weight as i32 + dw).min(params.w_max as i32);
            self.weight = mask(new_w, self.data_width);
        } else if pre_spike {
            let dw = (self.trace_post as i32 * params.a_minus.abs() as i32) >> self.fraction;
            let new_w = (self.weight as i32 - dw).max(params.w_min as i32);
            self.weight = mask(new_w, self.data_width);
        }
    }
}

/// Reward-modulated STDP synapse.
///
/// Eligibility trace accumulates Hebbian coincidences; weight update
/// fires only when a global reward signal arrives.
/// Izhikevich, Cerebral Cortex 17(10), 2007.
#[derive(Clone, Debug)]
pub struct RewardStdpSynapse {
    pub weight: f64,
    pub w_min: f64,
    pub w_max: f64,
    pub eligibility: f64,
    pub trace_decay: f64,
    pub anti_hebbian_scale: f64,
    pub learning_rate: f64,
}

impl RewardStdpSynapse {
    pub fn new(w: f64, w_min: f64, w_max: f64) -> Self {
        Self {
            weight: w,
            w_min,
            w_max,
            eligibility: 0.0,
            trace_decay: 0.95,
            anti_hebbian_scale: 0.5,
            learning_rate: 0.01,
        }
    }

    /// Accumulate eligibility trace from pre/post spike coincidence.
    pub fn step(&mut self, pre: bool, post: bool) {
        if pre && post {
            self.eligibility += 1.0;
        } else if pre && !post {
            self.eligibility -= self.anti_hebbian_scale;
        }
        self.eligibility *= self.trace_decay;
    }

    /// Apply reward signal: weight += lr * reward * eligibility.
    pub fn apply_reward(&mut self, reward: f64) {
        let update = self.learning_rate * reward * self.eligibility;
        self.weight = (self.weight + update).clamp(self.w_min, self.w_max);
    }
}

/// Static synapse with excitatory/inhibitory polarity.
#[derive(Clone, Debug)]
pub struct StaticSynapse {
    pub weight: f64,
    pub is_excitatory: bool,
    pub delay: u32,
}

impl StaticSynapse {
    pub fn new(weight: f64, is_excitatory: bool) -> Self {
        Self {
            weight: weight.abs(),
            is_excitatory,
            delay: 0,
        }
    }

    /// Compute post-synaptic current from pre-synaptic spike.
    pub fn transmit(&self, pre_spike: bool) -> f64 {
        if !pre_spike {
            return 0.0;
        }
        if self.is_excitatory {
            self.weight
        } else {
            -self.weight
        }
    }
}

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

    fn default_params() -> StdpParams {
        StdpParams {
            a_plus: 64,  // 0.25 in Q8.8
            a_minus: 48, // 0.1875 in Q8.8
            decay: 230,  // ~0.90 in Q8.8
            w_min: 0,
            w_max: 255,
        }
    }

    #[test]
    fn potentiation_increases_weight() {
        let mut syn = StdpSynapse::new(128, 16, 8);
        let params = default_params();
        // Multiple pre spikes to build trace
        for _ in 0..5 {
            syn.step(true, false, &params);
        }
        let w_before = syn.weight;
        // Post spike triggers LTP via accumulated pre-trace
        syn.step(false, true, &params);
        assert!(syn.weight > w_before, "LTP must increase weight");
    }

    #[test]
    fn depression_decreases_weight() {
        let mut syn = StdpSynapse::new(128, 16, 8);
        let params = default_params();
        // Multiple post spikes to build trace
        for _ in 0..5 {
            syn.step(false, true, &params);
        }
        let w_before = syn.weight;
        // Pre spike triggers LTD via accumulated post-trace
        syn.step(true, false, &params);
        assert!(syn.weight < w_before, "LTD must decrease weight");
    }

    // ── RewardStdpSynapse tests ─────────────────────────────────

    #[test]
    fn rstdp_positive_reward_potentiates() {
        let mut syn = RewardStdpSynapse::new(0.5, 0.0, 1.0);
        for _ in 0..10 {
            syn.step(true, true);
        }
        let w_before = syn.weight;
        syn.apply_reward(1.0);
        assert!(syn.weight > w_before);
    }

    #[test]
    fn rstdp_negative_reward_depresses() {
        let mut syn = RewardStdpSynapse::new(0.5, 0.0, 1.0);
        for _ in 0..10 {
            syn.step(true, true);
        }
        let w_before = syn.weight;
        syn.apply_reward(-1.0);
        assert!(syn.weight < w_before);
    }

    #[test]
    fn rstdp_weight_bounded() {
        let mut syn = RewardStdpSynapse::new(0.5, 0.0, 1.0);
        for _ in 0..100 {
            syn.step(true, true);
            syn.apply_reward(10.0);
        }
        assert!(syn.weight <= 1.0);
        assert!(syn.weight >= 0.0);
    }

    // ── StaticSynapse tests ───────────────────────────────────────

    #[test]
    fn static_excitatory() {
        let syn = StaticSynapse::new(0.5, true);
        assert!((syn.transmit(true) - 0.5).abs() < 1e-12);
        assert!((syn.transmit(false)).abs() < 1e-12);
    }

    #[test]
    fn static_inhibitory() {
        let syn = StaticSynapse::new(0.5, false);
        assert!((syn.transmit(true) + 0.5).abs() < 1e-12);
    }

    #[test]
    fn weight_stays_in_bounds() {
        let mut syn = StdpSynapse::new(0, 16, 8);
        let params = default_params();
        for _ in 0..200 {
            syn.step(true, false, &params);
        }
        assert!(syn.weight >= params.w_min, "weight below w_min");
        assert!(syn.weight <= params.w_max, "weight above w_max");

        let mut syn2 = StdpSynapse::new(255, 16, 8);
        for _ in 0..200 {
            syn2.step(false, true, &params);
        }
        assert!(syn2.weight >= params.w_min);
        assert!(syn2.weight <= params.w_max);
    }
}