vlib/math: Add a pure V backend for vlib/math (#11267)

vlib/math/log.v

@@ -0,0 +1,76 @@
+module math
+
+pub fn log_n(x f64, b f64) f64 {
+	y := log(x)
+	z := log(b)
+	return y / z
+}
+
+// log10 returns the decimal logarithm of x.
+// The special cases are the same as for log.
+pub fn log10(x f64) f64 {
+	return log(x) * (1.0 / ln10)
+}
+
+// log2 returns the binary logarithm of x.
+// The special cases are the same as for log.
+pub fn log2(x f64) f64 {
+	frac, exp := frexp(x)
+	// Make sure exact powers of two give an exact answer.
+	// Don't depend on log(0.5)*(1/ln2)+exp being exactly exp-1.
+	if frac == 0.5 {
+		return f64(exp - 1)
+	}
+	return log(frac) * (1.0 / ln2) + f64(exp)
+}
+
+pub fn log1p(x f64) f64 {
+	y := 1.0 + x
+	z := y - 1.0
+	return log(y) - (z - x) / y // cancels errors with IEEE arithmetic
+}
+
+// log_b returns the binary exponent of x.
+//
+// special cases are:
+// log_b(±inf) = +inf
+// log_b(0) = -inf
+// log_b(nan) = nan
+pub fn log_b(x f64) f64 {
+	if x == 0 {
+		return inf(-1)
+	}
+	if is_inf(x, 0) {
+		return inf(1)
+	}
+	if is_nan(x) {
+		return x
+	}
+	return f64(ilog_b_(x))
+}
+
+// ilog_b returns the binary exponent of x as an integer.
+//
+// special cases are:
+// ilog_b(±inf) = max_i32
+// ilog_b(0) = min_i32
+// ilog_b(nan) = max_i32
+pub fn ilog_b(x f64) int {
+	if x == 0 {
+		return min_i32
+	}
+	if is_nan(x) {
+		return max_i32
+	}
+	if is_inf(x, 0) {
+		return max_i32
+	}
+	return ilog_b_(x)
+}
+
+// ilog_b returns the binary exponent of x. It assumes x is finite and
+// non-zero.
+fn ilog_b_(x_ f64) int {
+	x, exp := normalize(x_)
+	return int((f64_bits(x) >> shift) & mask) - bias + exp
+}