fixed cos sin

2026-09-11 15:09:47 +03:00
parent adfae10938
commit 6d7fe7392a
10 changed files with 345 additions and 6 deletions
@@ -9,7 +9,6 @@
 /* Alternitive semntic: nearest upper power of two */
 static inline int com_bits_needed(uint32_t v) {
 #ifdef COM_DEF_COMPILE_MODERN
-  /* Use intrinsic, required for fast sqrt impl */
  return v == 0 ? 1 : 32 - __builtin_clz(v);
 #else
 #endif
@@ -1,8 +1,14 @@
+/*
+    Fixed point arithmetic.
+    https://github.com/howerj/q/blob/master/q.c
+*/
+
 #include "fixed.h"
-#include "bits.h"
 #include <assert.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <tgmath.h>

 // 4-bit LUT (16 entries) for the normalized range [0.5, 2.0)
 // It stores the initial guess scaled to Q16.16.
@@ -10,6 +16,22 @@ static const uint32_t com_fixed_sqrt_lut[16] = {
    46340, 49547, 52521, 55314, 57954, 60464, 62862, 65161,
    67373, 69508, 71572, 73572, 75514, 77402, 79240, 81033};

+/* Used for other quadrants as well as cosine eval, all from the same table. */
+/* Only fractional part is present, reducing the cache footprint. */
+const uint16_t com_fixed_sin_lut[128] = {
+    0,     804,   1608,  2412,  3215,  4018,  4821,  5622,  6423,  7223,  8022,
+    8819,  9616,  10410, 11204, 11995, 12785, 13573, 14359, 15142, 15923, 16702,
+    17479, 18253, 19024, 19792, 20557, 21319, 22078, 22833, 23586, 24334, 25079,
+    25820, 26557, 27291, 28020, 28745, 29465, 30181, 30893, 31600, 32302, 32999,
+    33692, 34379, 35061, 35738, 36409, 37075, 37736, 38390, 39039, 39682, 40319,
+    40950, 41575, 42194, 42806, 43412, 44011, 44603, 45189, 45768, 46340, 46906,
+    47464, 48015, 48558, 49095, 49624, 50146, 50660, 51166, 51665, 52155, 52639,
+    53114, 53581, 54040, 54491, 54933, 55368, 55794, 56212, 56621, 57022, 57414,
+    57797, 58172, 58538, 58895, 59243, 59583, 59913, 60235, 60547, 60850, 61144,
+    61429, 61705, 61971, 62228, 62475, 62714, 62942, 63162, 63371, 63571, 63762,
+    63943, 64115, 64276, 64428, 64571, 64703, 64826, 64939, 65043, 65136, 65220,
+    65294, 65358, 65412, 65457, 65491, 65516, 65531};
+
 com_fixed_t com_fixed_sqrt(com_fixed_t a) {
  // 1. Handle sign and edge cases
  assert(a >= 0);
@@ -18,11 +40,11 @@ com_fixed_t com_fixed_sqrt(com_fixed_t a) {

  // 2. Normalize input to the range [0.5, 2.0) to maximize LUT precision
  // clz = count leading zeros. On modern hardware, use __builtin_clz
-  int leading_zeros = com_bits_needed(a);
+  uint32_t leading_zeros = __builtin_clz(a);

  // Calculate how much we need to shift to place the highest bit properly
  // We want the value to land squarely within an optimal window
-  int shift = (31 - leading_zeros) - COM_FIXED_FRACBITS;
+  int32_t shift = (31 - leading_zeros) - COM_FIXED_FRACBITS;

  // Normalize shift to always be even so we can cleanly pull out 2^(shift/2)
  if (shift & 1)
@@ -69,3 +91,40 @@ void com_fixed_print(com_fixed_t a) {

  printf("%d.%04u\n", int_part, decimal_val);
 }
+
+void com_fixed_run_tests(void) {
+  com_fixed_print(com_fixed_sqrt(COM_FIXED_FRACUNIT * 2));
+  com_fixed_print(com_fixed_sqrt(COM_FIXED_FRACUNIT * 3));
+  com_fixed_print(com_fixed_sqrt(COM_FIXED_FRACUNIT * 4));
+  com_fixed_print(com_fixed_sqrt(COM_FIXED_FRACUNIT * 200));
+  com_fixed_print(com_fixed_sqrt(COM_FIXED_FRACUNIT * 1000));
+
+  com_fixed_print(COM_FIXED_PI);
+  com_fixed_print(com_fixed_div(COM_FIXED_PI + 234, COM_FIXED_PI2) << 2);
+
+  double max_sin_deviation = 0.0f;
+  com_fixed_t sin_accumulator = -COM_FIXED_PI * 4;
+  com_fixed_t sin_test_step = (COM_FIXED_PI << 1) >> 10;
+  for (int i = 0; i < 1024 * 32; ++i) {
+    com_fixed_t sin = com_fixed_sin(sin_accumulator);
+    double deviation = fabs(sin(com_fixed_as_float(sin_accumulator)) -
+                            com_fixed_as_float(sin));
+    if (deviation > max_sin_deviation)
+      max_sin_deviation = deviation;
+    sin_accumulator += sin_test_step;
+  }
+  printf("max sin deviation: %f\n", max_sin_deviation);
+
+  double max_cos_deviation = 0.0f;
+  com_fixed_t cos_accumulator = -COM_FIXED_PI * 4;
+  com_fixed_t cos_test_step = (COM_FIXED_PI << 1) >> 10;
+  for (int i = 0; i < 1024 * 32; ++i) {
+    com_fixed_t cos = com_fixed_cos(cos_accumulator);
+    double deviation = fabs(cos(com_fixed_as_float(cos_accumulator)) -
+                            com_fixed_as_float(cos));
+    if (deviation > max_cos_deviation)
+      max_cos_deviation = deviation;
+    cos_accumulator += cos_test_step;
+  }
+  printf("max cos deviation: %f\n", max_cos_deviation);
+}
@@ -7,12 +7,22 @@
 #define COM_FIXED_H

 #include <stdint.h>
+#include <stdio.h>

 #define COM_FIXED_FRACBITS 16
 #define COM_FIXED_FRACUNIT (1 << COM_FIXED_FRACBITS)
+#define COM_FIXED_FRACHALF (COM_FIXED_FRACUNIT >> 1)
+#define COM_FIXED_FRACQRTR (COM_FIXED_FRACHALF >> 1)
+
+#define COM_FIXED_PI (com_fixed_t)205887
+#define COM_FIXED_PI2 (com_fixed_t)411774
+#define COM_FIXED_PIHALF (com_fixed_t)102943
+#define COM_FIXED_PIONEANDAHALF (COM_FIXED_PI + COM_FIXED_PIHALF)

 typedef int32_t com_fixed_t;

+extern const uint16_t com_fixed_sin_lut[128];
+
 static inline com_fixed_t com_fixed_add(com_fixed_t a, com_fixed_t b) {
  return a + b;
 }
@@ -30,9 +40,61 @@ static inline com_fixed_t com_fixed_div(com_fixed_t a, com_fixed_t b) {
  return (com_fixed_t)(((int64_t)a << COM_FIXED_FRACBITS) / (int64_t)b);
 }

-/* Approximate square root using Newton-Raphson in 2 iterations over small LUT*/
+/* Is supposed to be only used for debugging, not in real code. */
+/* Because of that we default to maximum precision here. */
+static inline double com_fixed_as_float(com_fixed_t a) {
+  return (a >> COM_FIXED_FRACBITS) * (double)1.0 +
+         (a & 0xFFFF) * (double)(1.0 / COM_FIXED_FRACUNIT);
+}
+
+/* Approximated square root by Newton-Raphson in 2 iterations over small LUT. */
 com_fixed_t com_fixed_sqrt(com_fixed_t a);

+/* Approximated using LUT. */
+/* TODO: test whether linearly interpolated version could be default. */
+/*       things like camera movement can be jarring if done without.*/
+/* https://namoseley.wordpress.com/2015/07/26/sincos-generation-using-table-lookup-and-iterpolation/
+ */
+static inline com_fixed_t com_fixed_sin(com_fixed_t a) {
+  int32_t sign = 1;
+
+  /* Wrap to (-Pi2,+Pi2) */
+  com_fixed_t q = a % COM_FIXED_PI2;
+
+  /* Wrap to [0,+Pi2) */
+  if (q < 0)
+    q = COM_FIXED_PI2 + q;
+
+  /* Limit to [0,+1) */
+  q = com_fixed_div(q, COM_FIXED_PI2);
+
+  /* Handle cases of 3rd and 4th quadrant. */
+  if (q >= COM_FIXED_FRACHALF) {
+    q -= COM_FIXED_FRACHALF;
+    sign = -1;
+  }
+
+  /* Finally clculate the index into LUT. */
+  uint16_t const idx = q >= COM_FIXED_FRACQRTR ? 255 - (q >> 7) : q >> 7;
+
+  return com_fixed_sin_lut[idx] * sign;
+}
+
+/* Implemented over sin, as to share one single LUT. */
+/* Additionally, optimizer probably can collapse some math for combined sincos
+ * case. */
+static inline com_fixed_t com_fixed_cos(com_fixed_t a) {
+  return com_fixed_sin(a + COM_FIXED_PIHALF);
+}
+
+static inline void com_fixed_sincos(com_fixed_t a, com_fixed_t *restrict s,
+                                    com_fixed_t *restrict c) {
+  *s = com_fixed_sin(a);
+  *c = com_fixed_cos(a);
+}
+
 void com_fixed_print(com_fixed_t a);

+void com_fixed_run_tests(void);
+
 #endif
@@ -134,4 +134,54 @@ static inline com_mat_t com_mat_reoder(com_mat_t a) {
  return result;
 }

+/* TODO: move to .c file */
+/* Produces a view matrix needed for camera work. */
+static inline com_mat_t com_mat_look_at(com_vec_t pos, com_vec_t up,
+                                        com_vec_t target) {
+  com_vec_t const r = com_vec_nrm(com_vec_crs(target, up));
+  com_vec_t const u = com_vec_crs(r, target);
+
+  com_mat_t result;
+
+  result.a[0] = r.s.x;
+  result.a[1] = u.s.x;
+  result.a[2] = -target.s.x;
+  result.a[3] = 0;
+  result.a[4] = r.s.y;
+  result.a[5] = u.s.y;
+  result.a[6] = -target.s.y;
+  result.a[7] = 0;
+  result.a[8] = r.s.z;
+  result.a[9] = u.s.z;
+  result.a[10] = -target.s.z;
+  result.a[11] = 0;
+  result.a[12] = -com_vec_dot(r, pos);
+  result.a[13] = -com_vec_dot(u, pos);
+  result.a[14] = com_vec_dot(target, pos);
+  result.a[15] = COM_FIXED_FRACUNIT;
+
+  return result;
+}
+
+/* TODO: move to .c file */
+/* Produces a projection matrix needed for camera work. */
+static inline com_mat_t com_mat_perspective(uint16_t rwidth, uint16_t rheight,
+                                            com_fixed_t nearz, com_fixed_t farz,
+                                            com_fixed_t fov) {
+  com_mat_t result = {0};
+
+  // com_fixed_t const aspect = com_fixed_div(rwidth, rheight);
+
+  // const float f = 1.0f / tanf(camera->fov * 0.5f);
+  // const float fn = 1.0f / (CAMERA_NEAR_Z - camera->far_z);
+
+  // result.row[0].x = f / aspect;
+  // result.row[1].y = f;
+  // result.row[2].z = (CAMERA_NEAR_Z + camera->far_z) * fn;
+  // result.row[2].w = -1.0f;
+  // result.row[3].z = 2.0f * CAMERA_NEAR_Z * camera->far_z * fn;
+
+  // return result;
+}
+
 #endif
@@ -20,6 +20,12 @@ typedef union {
  } s;
 } com_vec_t;

+static inline com_vec_t com_vec_identity(void) {
+  return (com_vec_t){.s = {.x = COM_FIXED_FRACUNIT,
+                           .y = COM_FIXED_FRACUNIT,
+                           .z = COM_FIXED_FRACUNIT}};
+}
+
 static inline com_vec_t com_vec_add(com_vec_t a, com_vec_t b) {
  return (com_vec_t){.s = {.x = com_fixed_add(a.s.x, b.s.x),
                           .y = com_fixed_add(a.s.y, b.s.y),