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const std = @import("std");
pub fn main() !void {
try std.fmt.format(std.io.getStdOut().writer(), "day 9 pt1: {}\n", .{try solve_pt1(std.heap.page_allocator, puzzle_input)});
try std.fmt.format(std.io.getStdOut().writer(), "day 9 pt2: {}\n", .{try solve_pt2(std.heap.page_allocator, puzzle_input)});
}
fn solve_pt1(a: std.mem.Allocator, input: []const u8) !i64 {
var sum: i64 = 0;
var spl = std.mem.split(u8, input, "\n");
while (spl.next()) |line| {
const nums = try readnums(a, line);
defer nums.deinit();
const nxt = nums.getLast() + try predictNext(a, nums);
// std.log.warn("nxt {}", .{nxt});
sum += nxt;
}
return sum;
}
// note uses recursion!
fn predictNext(a: std.mem.Allocator, list: std.ArrayList(i64)) !i64 {
if (std.mem.allEqual(i64, list.items, 0)) {
return 0;
}
var sublist = std.ArrayList(i64).init(a);
defer sublist.deinit();
for (1..list.items.len) |ix| {
try sublist.append(list.items[ix] - list.items[ix - 1]);
}
const nxt_diff = try predictNext(a, sublist);
return sublist.getLast() + nxt_diff;
}
fn readnums(a: std.mem.Allocator, input: []const u8) !std.ArrayList(i64) {
var toks = std.mem.tokenize(u8, input, " ");
var res = std.ArrayList(i64).init(a);
while (toks.next()) |tok| {
try res.append(try std.fmt.parseInt(i64, tok, 10));
}
return res;
}
test "pt1" {
try std.testing.expectEqual(@as(i64, 114), try solve_pt1(std.testing.allocator, test_input));
}
fn solve_pt2(a: std.mem.Allocator, input: []const u8) !i64 {
var sum: i64 = 0;
var spl = std.mem.split(u8, input, "\n");
while (spl.next()) |line| {
const nums = try readnums(a, line);
defer nums.deinit();
const prv = nums.items[0] - try predictPrev(a, nums);
// std.log.warn("prv {}", .{prv});
sum += prv;
}
return sum;
}
// note uses recursion!
fn predictPrev(a: std.mem.Allocator, list: std.ArrayList(i64)) !i64 {
if (std.mem.allEqual(i64, list.items, 0)) {
return 0;
}
var sublist = std.ArrayList(i64).init(a);
defer sublist.deinit();
for (1..list.items.len) |ix| {
try sublist.append(list.items[ix] - list.items[ix - 1]);
}
return sublist.items[0] - try predictPrev(a, sublist);
}
test "pt2" {
try std.testing.expectEqual(@as(i64, 2), try solve_pt2(std.testing.allocator, test_input));
}
const test_input =
\\0 3 6 9 12 15
\\1 3 6 10 15 21
\\10 13 16 21 30 45
;
const puzzle_input = @embedFile("day9.in");
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