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diff --git a/linux/apps/advent2020/5b.mu b/linux/apps/advent2020/5b.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 apps/advent2020/5b.mu
+#   $ ./a.elf < input
+#
+# You'll need to register to download the 'input' file for yourself.
+
+fn main -> _/ebx: int {
+  var pass-storage: (array int 0x400)  # 1k ints
+  var pass/esi: (addr array int) <- address pass-storage
+  # phase 1: populate pass array
+  var line-storage: (stream byte 0x10)  # 16 bytes is enough
+  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-!=
+    # process line
+    var seat-id/eax: int <- convert-from-binary line
+    var dest/eax: (addr int) <- index pass, seat-id
+    copy-to *dest, 1
+    loop
+  }
+  # phase 2: skip empty seats
+  var i/eax: int <- copy 0
+  {
+    compare i, 0x400
+    break-if->=
+    var src/ecx: (addr int) <- index pass, i
+    compare *src, 0
+    break-if-!=
+    i <- increment
+    loop
+  }
+  # phase 3: skip non-empty seats
+  {
+    compare i, 0x400
+    break-if->=
+    var src/ecx: (addr int) <- index pass, i
+    compare *src, 0
+    break-if-=
+    i <- increment
+    loop
+  }
+  print-int32-decimal 0, i
+  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
+    var bit/edx: int <- copy 0
+    {
+      compare c, 0x42/B
+      break-if-!=
+      bit <- copy 1
+    }
+    {
+      compare c, 0x52/R
+      break-if-!=
+      bit <- copy 1
+    }
+    var bit-value/eax: int <- repeated-shift-left bit, i
+    result <- add bit-value
+    i <- decrement
+    loop
+  }
+  return result
+}