Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

vendor/serde_json/src/lexical/exponent.rs

@@ -0,0 +1,50 @@
+// Adapted from https://github.com/Alexhuszagh/rust-lexical.
+
+//! Utilities to calculate exponents.
+
+/// Convert usize into i32 without overflow.
+///
+/// This is needed to ensure when adjusting the exponent relative to
+/// the mantissa we do not overflow for comically-long exponents.
+#[inline]
+fn into_i32(value: usize) -> i32 {
+    if value > i32::max_value() as usize {
+        i32::max_value()
+    } else {
+        value as i32
+    }
+}
+
+// EXPONENT CALCULATION
+
+// Calculate the scientific notation exponent without overflow.
+//
+// For example, 0.1 would be -1, and 10 would be 1 in base 10.
+#[inline]
+pub(crate) fn scientific_exponent(
+    exponent: i32,
+    integer_digits: usize,
+    fraction_start: usize,
+) -> i32 {
+    if integer_digits == 0 {
+        let fraction_start = into_i32(fraction_start);
+        exponent.saturating_sub(fraction_start).saturating_sub(1)
+    } else {
+        let integer_shift = into_i32(integer_digits - 1);
+        exponent.saturating_add(integer_shift)
+    }
+}
+
+// Calculate the mantissa exponent without overflow.
+//
+// Remove the number of digits that contributed to the mantissa past
+// the dot, and add the number of truncated digits from the mantissa,
+// to calculate the scaling factor for the mantissa from a raw exponent.
+#[inline]
+pub(crate) fn mantissa_exponent(exponent: i32, fraction_digits: usize, truncated: usize) -> i32 {
+    if fraction_digits > truncated {
+        exponent.saturating_sub(into_i32(fraction_digits - truncated))
+    } else {
+        exponent.saturating_add(into_i32(truncated - fraction_digits))
+    }
+}