starkernel: item 4.3.3a -- Q48.16 trigonometry (Q.SIN/Q.COS)
Adds q48_reduce_angle() (range-reduce a signed Q48.16 angle into [-PI_Q48, PI_Q48] via one integer division plus a bounded fix-up loop) and q48_sin_approx/q48_cos_approx (Taylor series, terms n=3,5,7,9,11 for sin and n=2,4,6,8,10 for cos, early exit below 10). Q.SIN/Q.COS registered as FORTH words in q48_words.c, same pattern as Q.LOG/Q.EXP/Q.SQRT. Found mid-implementation: this codebase has two independent Q48.16 implementations -- src/word_source/q48_16_words.c (hosted/vendored) and src/starkernel/math/q48_16.c (kernel-only; the kernel build does not compile the former at all). The hosted build linked fine after the first pass; the kernel build failed with undefined references until the same two functions were added to both .c files and both q48_16.h headers (include/q48_16.h and include/starkernel/q48_16.h). Not fixed at the root -- Q.LOG/Q.EXP/Q.SQRT already had this same four-file duplication, unremarked until now -- just navigated correctly for this item. Verified live on amd64 via serial injection: sin/cos at 0, +-pi/2, pi, and 3pi (range-reduction across multiple turns) all match expected values within Taylor-series truncation error (<0.2%). All three architectures boot clean to ok> with the DoE completing; dict_hash identical across all three (0x291a660b05fa7b52). FABRIC.md item 4.3.3a marked done with full acceptance evidence.
This commit is contained in:
Robert Allan James
@@ -414,6 +414,117 @@ q48_16_t q48_sqrt_approx(q48_16_t q)
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return x;
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}
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/* ============================================================================
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* Approximation: Sine / Cosine (Integer-Only, Taylor Series)
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* ============================================================================
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*
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* FABRIC.md item 4.3.3a -- needed by the Console drawing fabric's
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* CIRCLE/ARC/ELLIPSE (item 4.3.3b). Radian input, same Taylor-series
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* approach as q48_exp_approx above.
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*
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* PI_Q48 = 205887 (pi * 65536, rounded). TWO_PI_Q48 is derived as
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* 2 * PI_Q48 rather than independently rounded, so the range-reduction
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* boundary at +-PI_Q48 is self-consistent (no seam).
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*/
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/**
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* @brief Reduce a signed Q48.16 angle into [-PI_Q48, PI_Q48].
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*
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* A single integer division on the raw Q48.16 representations gives the
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* correct (scale-independent) quotient of how many full 2*pi turns to
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* remove, then a bounded fix-up loop (at most one or two iterations)
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* handles the remainder landing just outside the target interval.
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*
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* @param x Signed angle in Q48.16 format (any magnitude)
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* @return Equivalent angle in Q48.16 format, within [-PI_Q48, PI_Q48]
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*/
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static int64_t q48_reduce_angle(int64_t x)
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{
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const int64_t PI_Q48 = 205887; /* pi * 65536, rounded */
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const int64_t TWO_PI_Q48 = 2 * PI_Q48; /* derived, not independently rounded */
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int64_t k = x / TWO_PI_Q48;
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x -= k * TWO_PI_Q48;
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while (x > PI_Q48) x -= TWO_PI_Q48;
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while (x < -PI_Q48) x += TWO_PI_Q48;
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return x;
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}
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/**
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* @brief Compute an approximation of @c sin(q) in Q48.16 format (radians).
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*
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* Range-reduces into [-pi, pi] via @c q48_reduce_angle(), then sums the
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* Taylor series @c x - x^3/3! + x^5/5! - ... for terms n = 3,5,7,9,11,
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* with an early exit when a term falls below 10 (sub-LSB in Q48.16). Sine
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* is odd (@c sin(-x) = -sin(x)), so the sign is factored out before the
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* series and reapplied to the result.
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*
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* @param q Angle in Q48.16 format (radians, any magnitude)
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* @return @c sin(q) in Q48.16 format
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*/
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q48_16_t q48_sin_approx(q48_16_t q)
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{
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int64_t x = q48_reduce_angle((int64_t)q);
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int negative = (x < 0);
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if (negative) x = -x;
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q48_16_t xu = (q48_16_t)x;
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q48_16_t x2 = q48_mul(xu, xu);
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q48_16_t term = xu;
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q48_16_t result = xu;
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int subtract = 1;
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for (int n = 3; n <= 11; n += 2) {
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term = q48_mul(term, x2);
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term = q48_div(term, q48_from_u64((uint64_t)(n * (n - 1))));
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result = subtract ? q48_sub(result, term) : q48_add(result, term);
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subtract = !subtract;
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if (term < 10) break;
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}
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return negative ? (q48_16_t)(0ULL - result) : result;
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}
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/**
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* @brief Compute an approximation of @c cos(q) in Q48.16 format (radians).
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*
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* Range-reduces into [-pi, pi] via @c q48_reduce_angle(), then sums the
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* Taylor series @c 1 - x^2/2! + x^4/4! - ... for terms n = 2,4,6,8,10,
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* with an early exit when a term falls below 10. Cosine is even
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* (@c cos(-x) = cos(x)), so no sign correction is needed -- @c x^2 already
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* absorbs the sign of @c x.
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*
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* @param q Angle in Q48.16 format (radians, any magnitude)
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* @return @c cos(q) in Q48.16 format
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*/
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q48_16_t q48_cos_approx(q48_16_t q)
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{
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int64_t x = q48_reduce_angle((int64_t)q);
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q48_16_t xu = (q48_16_t)(x < 0 ? -x : x);
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q48_16_t x2 = q48_mul(xu, xu);
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q48_16_t term = 65536; /* 1.0 in Q48.16 */
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q48_16_t result = term;
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int subtract = 1;
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for (int n = 2; n <= 10; n += 2) {
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term = q48_mul(term, x2);
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term = q48_div(term, q48_from_u64((uint64_t)(n * (n - 1))));
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result = subtract ? q48_sub(result, term) : q48_add(result, term);
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subtract = !subtract;
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if (term < 10) break;
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}
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return result;
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}
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/* ============================================================================
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* Diagnostic / Testing Utilities
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* ============================================================================
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@@ -24,6 +24,8 @@
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* Q.LOG ( q -- ln_q ) natural log, Newton-Raphson
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* Q.EXP ( q -- e^q ) Taylor series
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* Q.SQRT ( q -- sqrt_q ) Newton-Raphson
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* Q.SIN ( q -- sin_q ) Taylor series, radians, any magnitude
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* Q.COS ( q -- cos_q ) Taylor series, radians, any magnitude
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* Q.FROM-INT ( n -- q ) integer → Q48.16 (n << 16)
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* Q.TO-INT ( q -- n ) Q48.16 → integer (q >> 16, truncate)
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* Q.1 ( -- 65536 ) 1.0 in Q48.16
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@@ -115,6 +117,16 @@ static void q48_word_sqrt(VM *vm)
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q48_push(vm, q48_sqrt_approx(q48_pop(vm)));
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}
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static void q48_word_sin(VM *vm)
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{
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q48_push(vm, q48_sin_approx(q48_pop(vm)));
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}
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static void q48_word_cos(VM *vm)
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{
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q48_push(vm, q48_cos_approx(q48_pop(vm)));
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}
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/* ── conversions ─────────────────────────────────────────────────────── */
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static void q48_word_from_int(VM *vm)
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@@ -208,6 +220,8 @@ void register_q48_words(VM *vm)
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register_word(vm, "Q.LOG", q48_word_log);
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register_word(vm, "Q.EXP", q48_word_exp);
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register_word(vm, "Q.SQRT", q48_word_sqrt);
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register_word(vm, "Q.SIN", q48_word_sin);
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register_word(vm, "Q.COS", q48_word_cos);
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register_word(vm, "Q.FROM-INT", q48_word_from_int);
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register_word(vm, "Q.TO-INT", q48_word_to_int);
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register_word(vm, "Q.1", q48_word_one);
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