
(********************************************************************)
(*                                                                  *)
(*  leb128.s7i    Convert integers to and from LEB128 encoding.     *)
(*  Copyright (C) 2019  Thomas Mertes                               *)
(*                                                                  *)
(*  This file is part of the Seed7 Runtime Library.                 *)
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(*  The Seed7 Runtime Library is free software; you can             *)
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(*                                                                  *)
(********************************************************************)


(**
 *  Read a LEB128 encoded number from an ''inFile''.
 *  @param inFile File from which the LEB128 encoded number is read.
 *  @return the LEB128 encoded number as [[string]].
 *  @exception RANGE_ERROR If characters beyond '\255;' are present.
 *)
const func string: getLeb128 (inout file: inFile) is func
  result
    var string: resultStri is "";
  local
    var char: ch is ' ';
  begin
    ch := getc(inFile);
    while ch >= '\128;' and ch <= '\255;' do
      resultStri &:= ch;
      ch := getc(inFile);
    end while;
    if ch >= '\0;' and ch <= '\127;' then
      resultStri &:= ch;
    else
      raise RANGE_ERROR;
    end if;
  end func;


(**
 *  Decode a LEB128 encoded [[integer]] from a [[string]].
 *  @param stri String with an LEB128 encoded integer that starts at
 *              position ''pos''.
 *  @param pos When the function is called the LEB128 number starts at ''pos''.
 *             When the function is left ''pos'' refers to the character
 *             after the LEB128 number.
 *  @return the decoded LEB128 number.
 *  @exception RANGE_ERROR If characters beyond '\255;' are present.
 *  @exception OVERFLOW_ERROR If the result value cannot be represented
 *             with an integer.
 *)
const func integer: leb128ToInt (in string: stri, inout integer: pos) is func
  result
    var integer: anInt is 0;
  local
    var char: ch is ' ';
    var integer: lshift is 0;
  begin
    ch := stri[pos];
    while ch >= '\128;' and ch <= '\255;' do
      anInt +:= (ord(ch) - 128) << lshift;
      lshift +:= 7;
      incr(pos);
      ch := stri[pos];
    end while;
    if ch >= '\0;' and ch <= '\63;' then
      anInt +:= ord(ch) << lshift;
    elsif ch >= '\64;' and ch <= '\127;' then
      if lshift = 0 then
        anInt := ord(ch) - 128;
      else
        anInt := anInt + ((-2) << pred(lshift));
        anInt +:= (ord(ch) - 127) << lshift;
      end if;
    else
      raise RANGE_ERROR;
    end if;
    incr(pos);
  end func;


(**
 *  Decode a LEB128 encoded [[integer]] from a [[string]].
 *  @param stri String that starts with an LEB128 encoded integer.
 *  @return the decoded LEB128 number.
 *  @exception RANGE_ERROR If characters beyond '\255;' are present.
 *  @exception OVERFLOW_ERROR If the result value cannot be represented
 *             with an integer.
 *)
const func integer: leb128ToInt (in string: stri) is func
  result
    var integer: anInt is 0;
  local
    var integer: pos is 1;
  begin
    anInt := leb128ToInt(stri, pos);
  end func;


(**
 *  Decode an unsigned LEB128 encoded [[integer]] from a [[string]].
 *  @param stri String with an unsigned LEB128 encoded integer that starts at
 *              position ''pos''..
 *  @param pos When the function is called the LEB128 number starts at ''pos''.
 *             When the function is left ''pos'' refers to the character
 *             after the LEB128 number.
 *  @return the decoded unsigned LEB128 number.
 *  @exception RANGE_ERROR If characters beyond '\255;' are present.
 *  @exception OVERFLOW_ERROR If the result value cannot be represented
 *             with an integer.
 *)
const func integer: uLeb128ToInt (in string: stri, inout integer: pos) is func
  result
    var integer: anInt is 0;
  local
    var char: ch is ' ';
    var integer: lshift is 0;
  begin
    ch := stri[pos];
    while ch >= '\128;' and ch <= '\255;' do
      anInt +:= (ord(ch) - 128) << lshift;
      lshift +:= 7;
      incr(pos);
      ch := stri[pos];
    end while;
    if ch >= '\0;' and ch <= '\127;' then
      anInt +:= ord(ch) << lshift;
    else
      raise RANGE_ERROR;
    end if;
    incr(pos);
  end func;


(**
 *  Decode an unsigned LEB128 encoded [[integer]] from a [[string]].
 *  @param stri String that starts with an unsigned LEB128 encoded integer.
 *  @return the decoded unsigned LEB128 number.
 *  @exception RANGE_ERROR If characters beyond '\255;' are present.
 *  @exception OVERFLOW_ERROR If the result value cannot be represented
 *             with an integer.
 *)
const func integer: uLeb128ToInt (in string: stri) is func
  result
    var integer: anInt is 0;
  local
    var integer: pos is 1;
  begin
    anInt := uLeb128ToInt(stri, pos);
  end func;


(**
 *  Encode an [[integer]] with LEB128 encoding.
 *  @param number The number to be encoded.
 *  @return the LEB128 encoded number as [[string]].
 *)
const func string: leb128 (in var integer: number) is func
  result
    var string: leb128 is "";
  local
    var boolean: more is TRUE;
    var integer: aByte is 0;
  begin
    while more do
      aByte := number mod 128;
      number >>:= 7;
      # Sign bit of aByte is second high order bit (16#40) */
      if (number = 0 and aByte < 16#40) or
          (number = -1 and aByte >= 16#40) then
        more := FALSE;
      else
        aByte +:= 128;  # Set high order bit
      end if;
      leb128 &:= char(aByte);
    end while;
  end func;


(**
 *  Encode an [[integer]] with unsigned LEB128 encoding.
 *  @param number The number to be encoded.
 *  @return the unsigned LEB128 encoded number as [[string]].
 *)
const func string: uLeb128 (in var integer: number) is func
  result
    var string: uLeb128 is "";
  local
    var integer: aByte is 0;
  begin
    if number < 0 then
      raise RANGE_ERROR;
    else
      repeat
        aByte := number mod 128;
        number >>:= 7;
        if number <> 0 then
          aByte +:= 128;  # Set high order bit
        end if;
        uLeb128 &:= char(aByte);
      until number = 0;
    end if;
  end func;
