73 lines
2.7 KiB
C++
73 lines
2.7 KiB
C++
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// nisps/ml/init.hpp — weight initialization strategies.
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//
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// Three strategies are exposed:
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// - `uniform_init`: w ~ U[-1, 1]. Matches the legacy
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// `utils::gen_rand<T>()` baseline; produces high-magnitude pre-activations
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// that drive sigmoid output saturation.
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// - `xavier_init`: w ~ U[-1, 1] / sqrt(fan_in). Centered pre-activations,
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// better for sigmoid output layers.
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// - `spread_init`: linearly interpolates per-layer scale between the two.
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// scale = (1 - spread) + spread / sqrt(fan_in). spread=0 ⇒ uniform,
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// spread=1 ⇒ Xavier. This is the playground knob.
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//
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// All three operate on a flat row-major weight buffer of size fan_in*fan_out.
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// Biases are initialized separately and ALWAYS to zero — the legacy code
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// initialized biases to zero, and the playground spread parameter never
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// touches biases. We keep that.
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#pragma once
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#include <cmath>
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#include <cstddef>
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#include <span>
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#include "../core/rng.hpp"
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namespace nisps::ml {
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// Compute the spread-aware weight scale for one layer.
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// spread=0 → 1.0 (uniform [-1, 1])
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// spread=1 → 1/sqrt(fan_in) (Xavier)
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inline float spread_scale(float spread, std::size_t fan_in) noexcept {
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if (fan_in == 0u) return 1.f;
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const float inv_sqrt = 1.f / std::sqrt(static_cast<float>(fan_in));
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return (1.f - spread) + spread * inv_sqrt;
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}
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// Initialize one layer's weights (flat row-major) and biases.
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// weights: span of size fan_in * fan_out
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// biases: span of size fan_out (zeroed)
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// spread: see spread_scale
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// rng: state advanced; caller owns it
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inline void spread_init(std::span<float> weights,
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std::span<float> biases,
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std::size_t fan_in,
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float spread,
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Rng& rng) noexcept {
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const float scale = spread_scale(spread, fan_in);
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for (std::size_t i = 0; i < weights.size(); ++i) {
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weights[i] = rng.next_float_signed() * scale;
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}
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for (std::size_t i = 0; i < biases.size(); ++i) {
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biases[i] = 0.f;
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}
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}
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// Convenience aliases for the endpoints. Cheap to build on top of
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// spread_init; useful for tests that want to assert a specific regime.
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inline void uniform_init(std::span<float> weights,
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std::span<float> biases,
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std::size_t fan_in,
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Rng& rng) noexcept {
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spread_init(weights, biases, fan_in, 0.f, rng);
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}
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inline void xavier_init(std::span<float> weights,
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std::span<float> biases,
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std::size_t fan_in,
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Rng& rng) noexcept {
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spread_init(weights, biases, fan_in, 1.f, rng);
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}
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} // namespace nisps::ml
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