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day12.rs
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day12.rs
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//! # JSAbacusFramework.io
//!
//! ## Part One
//!
//! The utility [`iter_signed`] method extracts numbers from surrounding text and is used directly.
//!
//! ## Part Two
//!
//! We build a tiny custom JSON parser using a
//! [parser combinator](https://en.wikipedia.org/wiki/Parser_combinator) approach,
//! making some simplifying assumptions:
//! * The input is always well formed and does not contain any whitespace.
//! * Arrays and objects contain at least one item.
//! * We don't care about the content of strings, only if they equal "red" or not.
//!
//! Each parsing function returns a [`Result`] struct which has 3 fields:
//! * `next` The index of the character *after* this object. For example parsing "123," returns
//! a value of 3 for next.
//! * `ignore`: Only true for strings that exactly equals "red", false otherwise and always
//! false for numbers, arrays and objects.
//! * `value`: For numbers the literal value, for string zero, for arrays the sum of child
//! items, for objects the sum of child items if no "red" property is present, otherwise zero.
//!
//! [`iter_signed`]: crate::util::parse
use crate::util::parse::*;
const RED: &[u8] = b"red";
struct Result {
next: usize,
ignore: bool,
value: i32,
}
pub fn parse(input: &str) -> &str {
input
}
pub fn part1(input: &str) -> i32 {
input.iter_signed::<i32>().sum()
}
pub fn part2(input: &str) -> i32 {
parse_json(input.as_bytes(), 0).value
}
/// Parse JSON that has no whitespace.
fn parse_json(input: &[u8], start: usize) -> Result {
match input[start] {
b'[' => parse_array(input, start),
b'{' => parse_object(input, start),
b'"' => parse_string(input, start),
_ => parse_number(input, start),
}
}
/// Parse array assuming it contains at least one element.
fn parse_array(input: &[u8], start: usize) -> Result {
let mut index = start;
let mut total = 0;
while input[index] != b']' {
let Result { next, value, .. } = parse_json(input, index + 1);
index = next;
total += value;
}
Result { next: index + 1, ignore: false, value: total }
}
/// Parse object assuming it contains at least one key/value pair.
fn parse_object(input: &[u8], start: usize) -> Result {
let mut index = start;
let mut total = 0;
let mut ignore = false;
while input[index] != b'}' {
let Result { next: first, .. } = parse_json(input, index + 1);
let Result { next: second, ignore: red, value } = parse_json(input, first + 1);
index = second;
total += value;
ignore |= red;
}
Result { next: index + 1, ignore: false, value: if ignore { 0 } else { total } }
}
/// Parse a string evaluating only if it equals "red".
fn parse_string(input: &[u8], start: usize) -> Result {
let start = start + 1;
let mut end = start;
while input[end] != b'"' {
end += 1;
}
Result { next: end + 1, ignore: &input[start..end] == RED, value: 0 }
}
/// Parse an integer value.
fn parse_number(input: &[u8], start: usize) -> Result {
let mut end = start;
let mut neg = false;
let mut acc = 0;
if input[end] == b'-' {
neg = true;
end += 1;
}
while input[end].is_ascii_digit() {
acc = 10 * acc + (input[end] - b'0') as i32;
end += 1;
}
Result { next: end, ignore: false, value: if neg { -acc } else { acc } }
}