Import 'ryu' crate version 1.0.5
Bug: 163175424
Test: N/A
Change-Id: I7bda5411adccb26198c53ed1cb41fc6bb08468d7
diff --git a/src/s2f.rs b/src/s2f.rs
new file mode 100644
index 0000000..d9e0744
--- /dev/null
+++ b/src/s2f.rs
@@ -0,0 +1,207 @@
+use crate::common::*;
+use crate::f2s;
+use crate::f2s_intrinsics::*;
+use crate::parse::Error;
+#[cfg(feature = "no-panic")]
+use no_panic::no_panic;
+
+const FLOAT_EXPONENT_BIAS: usize = 127;
+
+fn floor_log2(value: u32) -> u32 {
+ 31_u32.wrapping_sub(value.leading_zeros())
+}
+
+#[cfg_attr(feature = "no-panic", no_panic)]
+pub fn s2f(buffer: &[u8]) -> Result<f32, Error> {
+ let len = buffer.len();
+ if len == 0 {
+ return Err(Error::InputTooShort);
+ }
+
+ let mut m10digits = 0;
+ let mut e10digits = 0;
+ let mut dot_index = len;
+ let mut e_index = len;
+ let mut m10 = 0u32;
+ let mut e10 = 0i32;
+ let mut signed_m = false;
+ let mut signed_e = false;
+
+ let mut i = 0;
+ if unsafe { *buffer.get_unchecked(0) } == b'-' {
+ signed_m = true;
+ i += 1;
+ }
+
+ while let Some(c) = buffer.get(i).copied() {
+ if c == b'.' {
+ if dot_index != len {
+ return Err(Error::MalformedInput);
+ }
+ dot_index = i;
+ i += 1;
+ continue;
+ }
+ if c < b'0' || c > b'9' {
+ break;
+ }
+ if m10digits >= 9 {
+ return Err(Error::InputTooLong);
+ }
+ m10 = 10 * m10 + (c - b'0') as u32;
+ if m10 != 0 {
+ m10digits += 1;
+ }
+ i += 1;
+ }
+
+ if let Some(b'e') | Some(b'E') = buffer.get(i) {
+ e_index = i;
+ i += 1;
+ match buffer.get(i) {
+ Some(b'-') => {
+ signed_e = true;
+ i += 1;
+ }
+ Some(b'+') => i += 1,
+ _ => {}
+ }
+ while let Some(c) = buffer.get(i).copied() {
+ if c < b'0' || c > b'9' {
+ return Err(Error::MalformedInput);
+ }
+ if e10digits > 3 {
+ // TODO: Be more lenient. Return +/-Infinity or +/-0 instead.
+ return Err(Error::InputTooLong);
+ }
+ e10 = 10 * e10 + (c - b'0') as i32;
+ if e10 != 0 {
+ e10digits += 1;
+ }
+ i += 1;
+ }
+ }
+
+ if i < len {
+ return Err(Error::MalformedInput);
+ }
+ if signed_e {
+ e10 = -e10;
+ }
+ e10 -= if dot_index < e_index {
+ (e_index - dot_index - 1) as i32
+ } else {
+ 0
+ };
+ if m10 == 0 {
+ return Ok(if signed_m { -0.0 } else { 0.0 });
+ }
+
+ if m10digits + e10 <= -46 || m10 == 0 {
+ // Number is less than 1e-46, which should be rounded down to 0; return
+ // +/-0.0.
+ let ieee = (signed_m as u32) << (f2s::FLOAT_EXPONENT_BITS + f2s::FLOAT_MANTISSA_BITS);
+ return Ok(f32::from_bits(ieee));
+ }
+ if m10digits + e10 >= 40 {
+ // Number is larger than 1e+39, which should be rounded to +/-Infinity.
+ let ieee = ((signed_m as u32) << (f2s::FLOAT_EXPONENT_BITS + f2s::FLOAT_MANTISSA_BITS))
+ | (0xff_u32 << f2s::FLOAT_MANTISSA_BITS);
+ return Ok(f32::from_bits(ieee));
+ }
+
+ // Convert to binary float m2 * 2^e2, while retaining information about
+ // whether the conversion was exact (trailing_zeros).
+ let e2: i32;
+ let m2: u32;
+ let mut trailing_zeros: bool;
+ if e10 >= 0 {
+ // The length of m * 10^e in bits is:
+ // log2(m10 * 10^e10) = log2(m10) + e10 log2(10) = log2(m10) + e10 + e10 * log2(5)
+ //
+ // We want to compute the FLOAT_MANTISSA_BITS + 1 top-most bits (+1 for
+ // the implicit leading one in IEEE format). We therefore choose a
+ // binary output exponent of
+ // log2(m10 * 10^e10) - (FLOAT_MANTISSA_BITS + 1).
+ //
+ // We use floor(log2(5^e10)) so that we get at least this many bits; better to
+ // have an additional bit than to not have enough bits.
+ e2 = floor_log2(m10)
+ .wrapping_add(e10 as u32)
+ .wrapping_add(log2_pow5(e10) as u32)
+ .wrapping_sub(f2s::FLOAT_MANTISSA_BITS + 1) as i32;
+
+ // We now compute [m10 * 10^e10 / 2^e2] = [m10 * 5^e10 / 2^(e2-e10)].
+ // To that end, we use the FLOAT_POW5_SPLIT table.
+ let j = e2
+ .wrapping_sub(e10)
+ .wrapping_sub(ceil_log2_pow5(e10))
+ .wrapping_add(f2s::FLOAT_POW5_BITCOUNT);
+ debug_assert!(j >= 0);
+ m2 = mul_pow5_div_pow2(m10, e10 as u32, j);
+
+ // We also compute if the result is exact, i.e.,
+ // [m10 * 10^e10 / 2^e2] == m10 * 10^e10 / 2^e2.
+ // This can only be the case if 2^e2 divides m10 * 10^e10, which in turn
+ // requires that the largest power of 2 that divides m10 + e10 is
+ // greater than e2. If e2 is less than e10, then the result must be
+ // exact. Otherwise we use the existing multiple_of_power_of_2 function.
+ trailing_zeros =
+ e2 < e10 || e2 - e10 < 32 && multiple_of_power_of_2_32(m10, (e2 - e10) as u32);
+ } else {
+ e2 = floor_log2(m10)
+ .wrapping_add(e10 as u32)
+ .wrapping_sub(ceil_log2_pow5(-e10) as u32)
+ .wrapping_sub(f2s::FLOAT_MANTISSA_BITS + 1) as i32;
+ let j = e2
+ .wrapping_sub(e10)
+ .wrapping_add(ceil_log2_pow5(-e10))
+ .wrapping_sub(1)
+ .wrapping_add(f2s::FLOAT_POW5_INV_BITCOUNT);
+ m2 = mul_pow5_inv_div_pow2(m10, -e10 as u32, j);
+ trailing_zeros = multiple_of_power_of_5_32(m10, -e10 as u32);
+ }
+
+ // Compute the final IEEE exponent.
+ let mut ieee_e2 = i32::max(0, e2 + FLOAT_EXPONENT_BIAS as i32 + floor_log2(m2) as i32) as u32;
+
+ if ieee_e2 > 0xfe {
+ // Final IEEE exponent is larger than the maximum representable; return
+ // +/-Infinity.
+ let ieee = ((signed_m as u32) << (f2s::FLOAT_EXPONENT_BITS + f2s::FLOAT_MANTISSA_BITS))
+ | (0xff_u32 << f2s::FLOAT_MANTISSA_BITS);
+ return Ok(f32::from_bits(ieee));
+ }
+
+ // We need to figure out how much we need to shift m2. The tricky part is
+ // that we need to take the final IEEE exponent into account, so we need to
+ // reverse the bias and also special-case the value 0.
+ let shift = if ieee_e2 == 0 { 1 } else { ieee_e2 as i32 }
+ .wrapping_sub(e2)
+ .wrapping_sub(FLOAT_EXPONENT_BIAS as i32)
+ .wrapping_sub(f2s::FLOAT_MANTISSA_BITS as i32);
+ debug_assert!(shift >= 0);
+
+ // We need to round up if the exact value is more than 0.5 above the value
+ // we computed. That's equivalent to checking if the last removed bit was 1
+ // and either the value was not just trailing zeros or the result would
+ // otherwise be odd.
+ //
+ // We need to update trailing_zeros given that we have the exact output
+ // exponent ieee_e2 now.
+ trailing_zeros &= (m2 & ((1_u32 << (shift - 1)) - 1)) == 0;
+ let last_removed_bit = (m2 >> (shift - 1)) & 1;
+ let round_up = last_removed_bit != 0 && (!trailing_zeros || ((m2 >> shift) & 1) != 0);
+
+ let mut ieee_m2 = (m2 >> shift).wrapping_add(round_up as u32);
+ if ieee_m2 == (1_u32 << (f2s::FLOAT_MANTISSA_BITS + 1)) {
+ // Due to how the IEEE represents +/-Infinity, we don't need to check
+ // for overflow here.
+ ieee_e2 += 1;
+ }
+ ieee_m2 &= (1_u32 << f2s::FLOAT_MANTISSA_BITS) - 1;
+ let ieee = ((((signed_m as u32) << f2s::FLOAT_EXPONENT_BITS) | ieee_e2 as u32)
+ << f2s::FLOAT_MANTISSA_BITS)
+ | ieee_m2;
+ Ok(f32::from_bits(ieee))
+}