Solved day 9 part 1.

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

inputs/day09.example.txt

@@ -0,0 +1 @@
+2333133121414131402

src/day09.zig

@@ -0,0 +1,92 @@
+const std = @import("std");
+const print = std.debug.print;
+const args = @import("lib/args.zig");
+const files = @import("lib/files.zig");
+
+inline fn normalize(c: u8) u8 {
+    return c - '0';
+}
+
+const FSEntryType = enum {
+    File,
+    EmptySpace,
+};
+
+const FSEntry = struct {
+    size: usize,
+    typ: FSEntryType,
+    // Only appicable to Files, may be best
+    // expressed as a union?
+    id: usize,
+};
+
+const FSPtr = struct {
+    entry: FSEntry,
+    pos: usize = 0,
+    idx: usize,
+};
+
+fn fsPtrFromIdx(dense: []u8, idx: usize) FSPtr {
+    return FSPtr{
+        .idx = idx,
+        .pos = 0,
+        .entry = FSEntry{
+            .size = normalize(dense[idx]),
+            .typ = if (idx % 2 == 0) .File else .EmptySpace,
+            .id = idx / 2,
+        },
+    };
+}
+
+pub fn main() !void {
+    const input_path = args.getFirstArg();
+    const input = try files.openForReading(input_path);
+    defer input.close();
+
+    var line_buf: [20000]u8 = undefined;
+    const dense = try files.readLine(input, &line_buf);
+    var left = fsPtrFromIdx(dense, 0);
+    var right = fsPtrFromIdx(dense, dense.len - 1);
+    right.pos = right.entry.size;
+
+    var checksum: u128 = 0;
+    var fsIdx: usize = 0;
+    while (true) {
+        if (left.idx > right.idx) {
+            break;
+        }
+        // Make sure that whatever we removed from right (if any)
+        // is honored.
+        if (left.idx == right.idx) {
+            left.entry.size = right.pos;
+        }
+
+        if (left.pos >= left.entry.size) {
+            left = fsPtrFromIdx(dense, left.idx + 1);
+            continue;
+        }
+        if (right.pos == 0) {
+            right = fsPtrFromIdx(dense, right.idx - 2);
+            right.pos = right.entry.size;
+            continue;
+        }
+        switch (left.entry.typ) {
+            .File => {
+                print("{d}", .{left.entry.id});
+                checksum += fsIdx * left.entry.id;
+                fsIdx += 1;
+                left.pos += 1;
+            },
+            .EmptySpace => {
+                print("{d}", .{right.entry.id});
+                checksum += fsIdx * right.entry.id;
+                left.pos += 1;
+                right.pos -= 1;
+                fsIdx += 1;
+            },
+        }
+    }
+    print("\n", .{});
+
+    print("Checksum: {any}\n", .{checksum});
+}