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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,155 @@ | ||
| use std::iter; | ||
|
|
||
| fn fast_parse_once(input: &[u8]) -> Option<(&[u8], u64)> { | ||
| let mut buf = input; | ||
| let mut output = 0; | ||
| while buf.first().is_some_and(u8::is_ascii_digit) { | ||
| output *= 10; | ||
| let (&first, rest) = buf.split_first().unwrap(); | ||
| output += u64::from(first - b'0'); | ||
| buf = rest; | ||
| } | ||
|
|
||
| if buf.len() != input.len() { | ||
| Some((buf, output)) | ||
| } else { | ||
| None | ||
| } | ||
| } | ||
|
|
||
| fn fast_parse_once_reverse(input: &[u8]) -> Option<(&[u8], Operand)> { | ||
| let mut buf = input; | ||
| let mut output = 0; | ||
| let mut unit = 1; | ||
| while buf.last().is_some_and(u8::is_ascii_digit) { | ||
| let (&last, rest) = buf.split_last().unwrap(); | ||
| output += u64::from(last - b'0') * unit; | ||
| unit *= 10; | ||
| buf = rest; | ||
| } | ||
|
|
||
| if buf.len() != input.len() { | ||
| Some((buf, Operand { value: output, bytes: &input[buf.len()..] })) | ||
| } else { | ||
| None | ||
| } | ||
| } | ||
|
|
||
| #[derive(Clone, Copy)] | ||
| struct Operand<'a> { | ||
| value: u64, | ||
| bytes: &'a [u8], | ||
| } | ||
|
|
||
| fn parse(input: &str) -> impl Iterator<Item = Line> { | ||
| input.lines().map(|line| { | ||
| let (operands, result) = fast_parse_once(line.as_bytes()).unwrap(); | ||
| Line { result, operands } | ||
| }) | ||
| } | ||
|
|
||
| impl<'a> Line<'a> { | ||
| fn operands_rev(&self) -> impl Iterator<Item = Operand> + Clone { | ||
| let mut operands = self.operands; | ||
| iter::from_fn(move || { | ||
| operands = operands.trim_ascii_end(); | ||
| let (rest, last) = fast_parse_once_reverse(operands)?; | ||
| operands = rest; | ||
| Some(last) | ||
| }) | ||
| } | ||
| } | ||
|
|
||
| struct Line<'a> { | ||
| result: u64, | ||
| operands: &'a [u8], | ||
| } | ||
|
|
||
| // Try iterating from the back. | ||
| // Break early if unable to reverse addition (negative) or multiplication (not divisible). | ||
| fn is_valid_reverse_recurse_p1<'a>( | ||
| result: u64, | ||
| mut operands: impl Iterator<Item = Operand<'a>> + Clone, | ||
| ) -> bool { | ||
| match operands.next() { | ||
| None => result == 0, // empty list | ||
| Some(Operand { value: last, .. }) => { | ||
| // Try addition | ||
| if let Some(intermediate) = result.checked_sub(last) { | ||
| if is_valid_reverse_recurse_p1(intermediate, operands.clone()) { | ||
| return true; | ||
| } | ||
| } | ||
|
|
||
| // Try multiplication | ||
| if result % last == 0 { | ||
| // operands are always nonzero | ||
| if is_valid_reverse_recurse_p1(result / last, operands) { | ||
| return true; | ||
| } | ||
| } | ||
|
|
||
| false | ||
| } | ||
| } | ||
| } | ||
|
|
||
| pub fn p1_reversed(input: String) -> u64 { | ||
| parse(&input) | ||
| .filter(|line| is_valid_reverse_recurse_p1(line.result, line.operands_rev())) | ||
| .map(|line| line.result) | ||
| .sum() | ||
| } | ||
|
|
||
| // Try iterating from the back. | ||
| // Break early if unable to reverse addition (negative) or multiplication (not divisible). | ||
| fn is_valid_reverse_recurse_p2<'a>( | ||
| result: u64, | ||
| mut operands: impl Iterator<Item = Operand<'a>> + Clone, | ||
| ) -> bool { | ||
| match operands.next() { | ||
| None => result == 0, // empty list | ||
| Some(last) => { | ||
| // Try addition | ||
| if let Some(intermediate) = result.checked_sub(last.value) { | ||
| if is_valid_reverse_recurse_p2(intermediate, operands.clone()) { | ||
| return true; | ||
| } | ||
| } | ||
|
|
||
| // Try concatenation | ||
| if let Some(intermediate) = strip_base10_suffix(result, last) { | ||
| if is_valid_reverse_recurse_p2(intermediate, operands.clone()) { | ||
| return true; | ||
| } | ||
| } | ||
|
|
||
| // Try multiplication | ||
| if result % last.value == 0 { | ||
| // no operands equal to 0 | ||
| if is_valid_reverse_recurse_p2(result / last.value, operands) { | ||
| return true; | ||
| } | ||
| } | ||
|
|
||
| false | ||
| } | ||
| } | ||
| } | ||
|
|
||
| fn strip_base10_suffix<'a>(long: u64, suffix: Operand<'a>) -> Option<u64> { | ||
| let remain = long - suffix.value; | ||
| let unit = 10u64.pow(suffix.bytes.len() as u32); | ||
| if remain % unit == 0 { | ||
| Some(remain / unit) | ||
| } else { | ||
| None | ||
| } | ||
| } | ||
|
|
||
| pub fn p2_reversed(input: String) -> u64 { | ||
| parse(&input) | ||
| .filter(|line| is_valid_reverse_recurse_p2(line.result, line.operands_rev())) | ||
| .map(|line| line.result) | ||
| .sum() | ||
| } |