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+# https://adventofcode.com/2020/day/3
+#
+# To run (on Linux):
+#   $ git clone https://github.com/akkartik/mu
+#   $ cd mu
+#   $ ./translate apps/advent2020/3a.mu
+#   $ ./a.elf < input
+#
+# You'll need to register to download the 'input' file for yourself.
+
+fn main -> _/ebx: int {
+  # represent trees in a 2D array of ints
+  # wasteful since each tree is just one bit
+  var trees-storage: (array int 0x2800)  # 10k ints
+  var trees/esi: (addr array int) <- address trees-storage
+  var trees-length/ecx: int <- copy 0
+  var num-rows: int
+  var width: int
+  # phase 1: parse each row of trees from stdin
+  {
+    var line-storage: (stream byte 0x40)  # 64 bytes
+    var line/edx: (addr stream byte) <- address line-storage
+    {
+      # read line from stdin
+      clear-stream line
+      read-line-from-real-keyboard line
+      # if line is empty (not even a newline), quit
+      var done?/eax: boolean <- stream-empty? line
+      compare done?, 0/false
+      break-if-!=
+      # wastefully recompute width on every line
+      # zero error-checking; we assume input lines are all equally long
+      copy-to width, 0
+      # turn each byte into a tree and append it
+      $main:line-loop: {
+        var done?/eax: boolean <- stream-empty? line
+        compare done?, 0/false
+        break-if-!=
+#?         print-int32-decimal 0, num-rows
+#?         print-string 0, " "
+#?         print-int32-decimal 0, width
+#?         print-string 0, "\n"
+        var dest/ebx: (addr int) <- index trees, trees-length
+        var c/eax: byte <- read-byte line
+        # newline comes only at end of line
+        compare c, 0xa/newline
+        break-if-=
+        # '#' = tree
+        compare c, 0x23/hash
+        {
+          break-if-!=
+          copy-to *dest, 1
+        }
+        # anything else = no tree
+        {
+          break-if-=
+          copy-to *dest, 0
+        }
+        increment width
+        trees-length <- increment
+        loop
+      }
+      increment num-rows
+      loop
+    }
+  }
+  # phase 2: compute
+  var product/edi: int <- copy 1
+  var result/eax: int <- num-trees-hit trees, width, num-rows, 1, 1
+  print-int32-decimal 0, result
+  print-string 0, " x "
+  product <- multiply result
+  var result/eax: int <- num-trees-hit trees, width, num-rows, 3, 1
+  print-int32-decimal 0, result
+  print-string 0, " x "
+  product <- multiply result
+  var result/eax: int <- num-trees-hit trees, width, num-rows, 5, 1
+  print-int32-decimal 0, result
+  print-string 0, " x "
+  product <- multiply result
+  var result/eax: int <- num-trees-hit trees, width, num-rows, 7, 1
+  print-int32-decimal 0, result
+  print-string 0, " x "
+  product <- multiply result
+  var result/eax: int <- num-trees-hit trees, width, num-rows, 1, 2
+  print-int32-decimal 0, result
+  print-string 0, " = "
+  product <- multiply result
+  print-int32-hex 0, product
+  print-string 0, "\n"
+  return 0
+}
+
+fn num-trees-hit trees: (addr array int), width: int, num-rows: int, right: int, down: int -> _/eax: int {
+#?   print-string 0, "== "
+#?   print-int32-decimal 0, right
+#?   print-string 0, " "
+#?   print-int32-decimal 0, down
+#?   print-string 0, "\n"
+  var row/ecx: int <- copy 0
+  var col/edx: int <- copy 0
+  var num-trees-hit/edi: int <- copy 0
+  {
+    compare row, num-rows
+    break-if->=
+#?     print-int32-decimal 0, col
+#?     print-string 0, "\n"
+    var curr/eax: int <- index2d trees, row, col, width
+    compare curr, 0
+    {
+      break-if-=
+      num-trees-hit <- increment
+    }
+    col <- add right
+    row <- add down
+    loop
+  }
+  return num-trees-hit
+}
+
+fn index2d _arr: (addr array int), _row: int, _col: int, width: int -> _/eax: int {
+  # handle repeating columns of trees
+  var dummy/eax: int <- copy 0
+  var col/edx: int <- copy 0
+  dummy, col <- integer-divide _col, width
+#?   print-string 0, "  "
+#?   print-int32-decimal 0, col
+#?   print-string 0, "\n"
+  # compute index
+  var index/eax: int <- copy _row
+  index <- multiply width
+  index <- add col
+  # look up array
+  var arr/esi: (addr array int) <- copy _arr
+  var src/eax: (addr int) <- index arr, index
+  return *src
+}