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diff --git a/apps/advent2020/5a.mu b/apps/advent2020/5a.mu
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+# https://adventofcode.com/2020/day/5
+#
+# To run (on Linux):
+#   $ git clone https://github.com/akkartik/mu
+#   $ cd mu
+#   $ ./translate_mu apps/advent2020/5a.mu
+#   $ ./a.elf < input
+#
+# You'll need to register to download the 'input' file for yourself.
+
+fn main -> _/ebx: int {
+  var line-storage: (stream byte 0x10)  # 16 bytes is enough
+  var line/edx: (addr stream byte) <- address line-storage
+  var max-seat-id/edi: int <- copy 0
+  {
+    # read line from stdin
+    clear-stream line
+    read-line-from-real-keyboard line
+    print-stream-to-real-screen line
+    # if line is empty (not even a newline), quit
+    var done?/eax: boolean <- stream-empty? line
+    compare done?, 0  # false
+    break-if-!=
+    # process line
+    var seat-id/eax: int <- convert-from-binary line
+    compare seat-id, max-seat-id
+    {
+      break-if-<=
+      max-seat-id <- copy seat-id
+    }
+    loop
+  }
+  print-int32-decimal 0, max-seat-id
+  print-string 0, "\n"
+  return 0
+}
+
+fn convert-from-binary in: (addr stream byte) -> _/eax: int {
+  var result/edi: int <- copy 0
+  var i/ecx: int <- copy 9  # loop counter and also exponent
+  {
+    compare i, 0
+    break-if-<
+    var c/eax: byte <- read-byte in
+#?     print-string 0, "char: "
+#?     {
+#?       var c2/eax: int <- copy c
+#?       print-int32-hex 0, c2
+#?     }
+#?     print-string 0, "\n"
+    var bit/edx: int <- copy 0
+    {
+      compare c, 0x42  # 'B'
+      break-if-!=
+      bit <- copy 1
+    }
+    {
+      compare c, 0x52  # 'R'
+      break-if-!=
+      bit <- copy 1
+    }
+#?     print-string 0, "bit: "
+#?     print-int32-decimal 0, bit
+#?     print-string 0, "\n"
+    var bit-value/eax: int <- repeated-shift-left bit, i
+#?     print-string 0, "bit value: "
+#?     print-int32-decimal 0, bit-value
+#?     print-string 0, "\n"
+    result <- add bit-value
+#?     print-string 0, "result: "
+#?     print-int32-decimal 0, result
+#?     print-string 0, "\n"
+    i <- decrement
+    loop
+  }
+  print-int32-decimal 0, result
+  print-string 0, "\n"
+  return result
+}