2023 Day 3

2023/day03.nim

@@ -0,0 +1,130 @@
+import std/sets
+import std/enumerate
+import std/tables
+import std/strutils
+
+const digits = "0123456789"
+
+type Coord = tuple
+  r: int
+  c: int
+
+type Slice = tuple
+  lo: Coord
+  hi: Coord
+
+type Grid = object
+  rows: int
+  cols: int
+  table: Table[Coord, char]
+  numbers: seq[Slice]
+  gears: seq[Coord]
+
+proc parseGrid(text: string): Grid =
+  result.table = initTable[Coord, char]()
+
+  let lines = text.strip.splitLines()
+  result.rows = lines.len
+  result.cols = lines[0].len
+
+  for (r, line) in enumerate(lines):
+    var num: Slice
+    var in_num = false
+
+    for (c, sym) in enumerate(line):
+      result.table[(r, c)] = sym
+
+      if sym == '*':
+        result.gears.add((r, c))
+
+      if sym in digits:
+        if in_num:
+          num.hi = (r, c)
+        else:
+          num.lo = (r, c)
+          num.hi = (r, c)
+        in_num = true
+        continue
+
+      if in_num:
+        result.numbers.add(num)
+        in_num = false
+
+    if in_num:
+      result.numbers.add(num)
+
+
+func neighbors(p: Coord): seq[Coord] =
+  for (dr, dc) in [
+    (-1, -1),
+    (-1, 0),
+    (-1, +1),
+    (0, -1),
+    # (0, 0)
+    (0, +1),
+    (+1, -1),
+    (+1, 0),
+    (+1, +1),
+  ]:
+    result.add((p.r + dr, p.c + dc))
+
+proc grid_sym(grid: Grid, p: Coord): char =
+  if 0 <= p.r and p.r < grid.rows and 0 <= p.c and p.c < grid.cols:
+    grid.table[p]
+  else:
+    '.'
+
+proc grid_slice(grid: Grid, s: Slice): string =
+  for r in s.lo.r .. s.hi.r:
+    for c in s.lo.c .. s.hi.c:
+      result.add(grid_sym(grid, (r, c)))
+
+proc has_neighbor(grid: Grid, s: Slice): bool =
+  let r = s.lo.r
+  for c in s.lo.c .. s.hi.c:
+    for n in neighbors((r, c)):
+      if n.r == r and s.lo.c <= n.c and n.c <= s.hi.c:
+        continue
+
+      let nsym = grid_sym(grid, n)
+      if nsym != '.':
+        return true
+  return false
+
+proc part1(): int =
+  let text = readFile("input/day03.txt")
+  let grid = parseGrid(text)
+
+  for num in grid.numbers:
+    assert(num.lo.r == num.hi.r)
+
+    if grid.has_neighbor(num):
+      result += grid_slice(grid, num).parseInt
+
+proc get_containing(grid: Grid, p: Coord): Slice =
+  for num in grid.numbers:
+    if p.r == num.lo.r and num.lo.c <= p.c and p.c <= num.hi.c:
+      return num
+
+proc gear_neighbors(grid: Grid, p: Coord): HashSet[Slice] =
+  for n in p.neighbors:
+    if grid.grid_sym(n)in digits:
+      result.incl(grid.get_containing(n))
+
+proc part2(): int =
+  let text = readFile("input/day03.txt")
+  let grid = parseGrid(text)
+
+  for gear in grid.gears:
+    let nums = grid.gear_neighbors(gear)
+    if nums.len != 2:
+      continue
+
+    var ratio = 1
+    for s in nums:
+      ratio *= grid.grid_slice(s).parseInt
+
+    result += ratio
+
+echo part1()
+echo part2()