IMPLEMENTATION MODULE Utf8;

PROCEDURE Decode (s : ARRAY OF CHAR; lo : CARDINAL;
                  VAR cp : UCHAR; VAR next : CARDINAL) : BOOLEAN;
  VAR b0, b1, b2, b3, v : INTEGER;
    i : CARDINAL;

  PROCEDURE Cont (VAR b : INTEGER) : BOOLEAN;
  (* consume a continuation byte (0x80..0xBF), or fail *)
    BEGIN
      IF i > HIGH(s) THEN RETURN FALSE END;
      b := ORD(s[i]); INC(i);
      RETURN (b >= 128) AND (b <= 191)
    END Cont;

  BEGIN
    cp := UCHR(0);
    next := lo;
    IF lo > HIGH(s) THEN RETURN FALSE END;
    b0 := ORD(s[lo]); i := lo + 1; v := -1;
    IF b0 <= 127 THEN
      v := b0
    ELSIF (b0 >= 194) AND (b0 <= 223) THEN
      IF Cont(b1) THEN v := ((b0 - 192) * 64) + (b1 - 128) END
    ELSIF (b0 >= 224) AND (b0 <= 239) THEN
      IF Cont(b1) AND Cont(b2) THEN
        v := ((b0 - 224) * 4096) + ((b1 - 128) * 64) + (b2 - 128);
        IF v < 2048 THEN v := -1 END
      END
    ELSIF (b0 >= 240) AND (b0 <= 244) THEN
      IF Cont(b1) AND Cont(b2) AND Cont(b3) THEN
        v := ((b0 - 240) * 262144) + ((b1 - 128) * 4096)
             + ((b2 - 128) * 64) + (b3 - 128);
        IF v < 65536 THEN v := -1 END
      END
    ELSE
      v := -1
    END;
    IF v < 0 THEN RETURN FALSE END;
    IF (v >= 55296) AND (v <= 57343) THEN RETURN FALSE END;
    IF v > 1114111 THEN RETURN FALSE END;
    cp := UCHR(v);
    next := i;
    RETURN TRUE
  END Decode;

PROCEDURE Encode (cp : UCHAR; VAR buf : ARRAY OF CHAR) : CARDINAL;
  VAR v : INTEGER;
    n : CARDINAL;
  BEGIN
    v := UORD(cp);
    IF (v >= 0) AND (v <= 127) THEN
      buf[0] := CHR(v); n := 1
    ELSIF (v >= 128) AND (v <= 2047) THEN
      buf[0] := CHR(192 + (v DIV 64));
      buf[1] := CHR(128 + (v MOD 64));
      n := 2
    ELSIF (v >= 2048) AND (v <= 65535) THEN
      buf[0] := CHR(224 + (v DIV 4096));
      buf[1] := CHR(128 + ((v DIV 64) MOD 64));
      buf[2] := CHR(128 + (v MOD 64));
      n := 3
    ELSIF (v >= 65536) AND (v <= 1114111) THEN
      buf[0] := CHR(240 + (v DIV 262144));
      buf[1] := CHR(128 + ((v DIV 4096) MOD 64));
      buf[2] := CHR(128 + ((v DIV 64) MOD 64));
      buf[3] := CHR(128 + (v MOD 64));
      n := 4
    ELSE
      n := 0
    END;
    RETURN n
  END Encode;

END Utf8.