1 files changed, 83 insertions, 0 deletions
diff --git a/third_party/rust/serde_json/src/lexical/parse.rs b/third_party/rust/serde_json/src/lexical/parse.rs
new file mode 100644
index 0000000000..e3d7f1e871
--- /dev/null
+++ b/third_party/rust/serde_json/src/lexical/parse.rs
@@ -0,0 +1,83 @@
+// Adapted from https://github.com/Alexhuszagh/rust-lexical.
+
+use super::algorithm::*;
+use super::bhcomp::*;
+use super::digit::*;
+use super::exponent::*;
+use super::num::*;
+
+// PARSERS
+// -------
+
+/// Parse float for which the entire integer and fraction parts fit into a 64
+/// bit mantissa.
+pub fn parse_concise_float<F>(mantissa: u64, mant_exp: i32) -> F
+where
+    F: Float,
+{
+    if let Some(float) = fast_path(mantissa, mant_exp) {
+        return float;
+    }
+
+    // Moderate path (use an extended 80-bit representation).
+    let truncated = false;
+    let (fp, valid) = moderate_path::<F>(mantissa, mant_exp, truncated);
+    if valid {
+        return fp.into_float::<F>();
+    }
+
+    let b = fp.into_downward_float::<F>();
+    if b.is_special() {
+        // We have a non-finite number, we get to leave early.
+        return b;
+    }
+
+    // Slow path, fast path didn't work.
+    let mut buffer = itoa::Buffer::new();
+    let integer = buffer.format(mantissa).as_bytes();
+    let fraction = &[];
+    bhcomp(b, integer, fraction, mant_exp)
+}
+
+/// Parse float from extracted float components.
+///
+/// * `integer`     - Slice containing the integer digits.
+/// * `fraction`    - Slice containing the fraction digits.
+/// * `exponent`    - Parsed, 32-bit exponent.
+///
+/// Precondition: The integer must not have leading zeros.
+pub fn parse_truncated_float<F>(integer: &[u8], mut fraction: &[u8], exponent: i32) -> F
+where
+    F: Float,
+{
+    // Trim trailing zeroes from the fraction part.
+    while fraction.last() == Some(&b'0') {
+        fraction = &fraction[..fraction.len() - 1];
+    }
+
+    // Calculate the number of truncated digits.
+    let mut truncated = 0;
+    let mut mantissa: u64 = 0;
+    let mut iter = integer.iter().chain(fraction);
+    for &c in &mut iter {
+        mantissa = match add_digit(mantissa, to_digit(c).unwrap()) {
+            Some(v) => v,
+            None => {
+                truncated = 1 + iter.count();
+                break;
+            }
+        };
+    }
+
+    let mant_exp = mantissa_exponent(exponent, fraction.len(), truncated);
+    let is_truncated = true;
+
+    fallback_path(
+        integer,
+        fraction,
+        mantissa,
+        exponent,
+        mant_exp,
+        is_truncated,
+    )
+}