mdscrub: residual html, control characters, headings, page markers

crates/textkit/src/number.rs

@@ -0,0 +1,91 @@
+//! Reading numbers out of text the way Python reads them.
+
+/// The value of a run of decimal digits, Unicode included.
+///
+/// Digits are matched by the Unicode class in the patterns these tools port,
+/// and Python's `int()` accepts every decimal digit, not just the ASCII ones
+/// — so `page٤٢` is page forty-two there and has to be here too. Non-digits
+/// are skipped, and a number too large for the field saturates rather than
+/// wrapping.
+pub fn decimal(text: &str) -> u32 {
+    let mut value: u32 = 0;
+    for digit in text.chars().filter_map(digit_value) {
+        value = value.saturating_mul(10).saturating_add(digit);
+    }
+    value
+}
+
+/// What one decimal digit is worth, in any script.
+///
+/// [`char::to_digit`] knows only the ASCII digits. Every Unicode decimal
+/// block, though, runs zero to nine in consecutive codepoints, so counting
+/// back to the first non-digit gives the value without a lookup table.
+fn digit_value(c: char) -> Option<u32> {
+    if let Some(digit) = c.to_digit(10) {
+        return Some(digit);
+    }
+    if !c.is_numeric() {
+        return None;
+    }
+    let code = c as u32;
+    let mut value = 9;
+    for back in 1..=9 {
+        let previous = code.checked_sub(back).and_then(char::from_u32);
+        match previous {
+            Some(previous) if previous.is_numeric() => continue,
+            _ => {
+                value = back - 1;
+                break;
+            }
+        }
+    }
+    // Confirm it really is a run of ten. Fractions and the other numeric
+    // characters sit in short runs, and would otherwise be read as digits:
+    // `½` follows `¼`, which would make it a one.
+    let zero = code - value;
+    let ten = (zero..zero + 10).all(|code| char::from_u32(code).is_some_and(char::is_numeric));
+    ten.then_some(value)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn ascii_digits_read_as_themselves() {
+        assert_eq!(decimal("<!-- Page 412 -->"), 412);
+    }
+
+    #[test]
+    fn other_decimal_digits_read_too() {
+        assert_eq!(decimal("٤٢"), 42);
+        assert_eq!(decimal("４２"), 42);
+    }
+
+    #[test]
+    fn every_digit_of_a_script_is_worth_what_it_looks_like() {
+        for (zero, name) in [('\u{660}', "arabic-indic"), ('\u{ff10}', "fullwidth")] {
+            for offset in 0..10u32 {
+                let digit = char::from_u32(zero as u32 + offset).expect("a digit");
+                assert_eq!(digit_value(digit), Some(offset), "{name} {digit}");
+            }
+        }
+    }
+
+    #[test]
+    fn a_character_that_is_not_a_digit_is_worth_nothing() {
+        for c in ['x', ' ', '-', '½'] {
+            assert_eq!(digit_value(c), None, "{c}");
+        }
+    }
+
+    #[test]
+    fn nothing_reads_as_zero() {
+        assert_eq!(decimal("page"), 0);
+    }
+
+    #[test]
+    fn an_absurd_number_saturates_rather_than_wrapping() {
+        assert_eq!(decimal(&"9".repeat(40)), u32::MAX);
+    }
+}