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+# Helpers for Unicode.
+#
+# Mu has no characters, only code points and graphemes.
+# Code points are the indivisible atoms of text streams.
+#   https://en.wikipedia.org/wiki/Code_point
+# Graphemes are the smallest self-contained unit of text.
+# Graphemes may consist of multiple code points.
+#
+# Mu graphemes are always represented in utf-8, and they are required to fit
+# in 4 bytes.
+#
+# Mu doesn't currently support combining code points, or graphemes made of
+# multiple code points. One day we will.
+
+# transliterated from tb_utf8_unicode_to_char in https://github.com/nsf/termbox
+# https://wiki.tcl-lang.org/page/UTF%2D8+bit+by+bit explains the algorithm
+#
+# The day we want to support combining characters, this function will need to
+# take multiple code points. Or something.
+fn to-grapheme in: code-point -> out/eax: grapheme {
+$to-grapheme:body: {
+  var c/eax: int <- copy in
+  var num-trailers/ecx: int <- copy 0
+  var first/edx: int <- copy 0
+  $to-grapheme:compute-length: {
+    # single byte: just return it
+    compare c, 0x7f
+    {
+      break-if->
+      out <- copy c
+      break $to-grapheme:body
+    }
+    # 2 bytes
+    compare c, 0x7ff
+    {
+      break-if->
+      num-trailers <- copy 1
+      first <- copy 0xc0
+      break $to-grapheme:compute-length
+    }
+    # 3 bytes
+    compare c, 0xffff
+    {
+      break-if->
+      num-trailers <- copy 2
+      first <- copy 0xe0
+      break $to-grapheme:compute-length
+    }
+    # 4 bytes
+    compare c, 0x1fffff
+    {
+      break-if->
+      num-trailers <- copy 3
+      first <- copy 0xf0
+      break $to-grapheme:compute-length
+    }
+    # more than 4 bytes: unsupported
+    compare c, 0x1fffff
+    {
+      break-if->
+      print-string-to-real-screen "unsupported code point "
+      print-int32-hex-to-real-screen c
+      print-string-to-real-screen "\n"
+      var exit-status/ebx: int <- copy 1
+      syscall_exit
+    }
+  }
+  # emit trailer bytes, 6 bits from 'in', first two bits '10'
+  var result/edi: int <- copy 0
+  {
+    compare num-trailers, 0
+    break-if-<=
+    var tmp/esi: int <- copy c
+    tmp <- and 0x3f
+    tmp <- or 0x80
+    result <- shift-left 8
+    result <- or tmp
+    # update loop state
+    c <- shift-right 6
+    num-trailers <- decrement
+    loop
+  }
+  # emit engine
+  result <- shift-left 8
+  result <- or c
+  result <- or first
+  #
+  out <- copy result
+}
+}
+
+# single-byte code point have identical graphemes
+fn test-to-grapheme-single-byte {
+  var in-int/ecx: int <- copy 0
+  {
+    compare in-int, 0x7f
+    break-if->
+    var in/eax: code-point <- copy in-int
+    var out/eax: grapheme <- to-grapheme in
+    var out-int/eax: int <- copy out
+    check-ints-equal out-int, in-int, "F - test-to-grapheme-single-byte"
+    in-int <- increment
+    loop
+  }
+}
+
+                                                              # byte       | byte      | byte      | byte
+# smallest 2-byte utf-8
+fn test-to-grapheme-two-bytes-min {
+  var in/eax: code-point <- copy 0x80                         #                                 10     00-0000
+  var out/eax: grapheme <- to-grapheme in
+  var out-int/eax: int <- copy out
+  check-ints-equal out-int, 0x80c2, "F - to-grapheme/2a"      #                         110 0-0010  10 00-0000
+}
+
+# largest 2-byte utf-8
+fn test-to-grapheme-two-bytes-max {
+  var in/eax: code-point <- copy 0x7ff                        #                             1-1111     11-1111
+  var out/eax: grapheme <- to-grapheme in
+  var out-int/eax: int <- copy out
+  check-ints-equal out-int, 0xbfdf, "F - to-grapheme/2b"      #                         110 1-1111  10 11-1111
+}
+
+# smallest 3-byte utf-8
+fn test-to-grapheme-three-bytes-min {
+  var in/eax: code-point <- copy 0x800                        #                            10-0000     00-0000
+  var out/eax: grapheme <- to-grapheme in
+  var out-int/eax: int <- copy out
+  check-ints-equal out-int, 0x80a0e0, "F - to-grapheme/3a"    #              1110 0000  10 10-0000  10 00-0000
+}
+
+# largest 3-byte utf-8
+fn test-to-grapheme-three-bytes-max {
+  var in/eax: code-point <- copy 0xffff                       #                   1111     11-1111     11-1111
+  var out/eax: grapheme <- to-grapheme in
+  var out-int/eax: int <- copy out
+  check-ints-equal out-int, 0xbfbfef, "F - to-grapheme/3b"    #              1110 1111  10 11-1111  10 11-1111
+}
+
+# smallest 4-byte utf-8
+fn test-to-grapheme-four-bytes-min {
+  var in/eax: code-point <- copy 0x10000                      #                 1-0000     00-0000     00-0000
+  var out/eax: grapheme <- to-grapheme in
+  var out-int/eax: int <- copy out
+  check-ints-equal out-int, 0x808090f0, "F - to-grapheme/4a"  # 1111-0 000  10 01-0000  10 00-0000  10 00-0000
+}
+
+# largest 4-byte utf-8
+fn test-to-grapheme-four-bytes-max {
+  var in/eax: code-point <- copy 0x1fffff                     #        111     11-1111     11-1111     11-1111
+  var out/eax: grapheme <- to-grapheme in
+  var out-int/eax: int <- copy out
+  check-ints-equal out-int, 0xbfbfbff7, "F - to-grapheme/4b"  # 1111-0 111  10 11-1111  10 11-1111  10 11-1111
+}
+
+# To run all tests, uncomment this and run:
+#   $ ./translate_mu  &&  ./a.elf
+#? fn main -> r/ebx: int {
+#?   run-tests
+#?   r <- copy 0
+#? }