; Compare two files without asking either one to fit in Data Memory.
;
; Each occupied block is read into its own 256-byte buffer. The byte count returned for
; the block is compared before its contents, and only those bytes are examined: bytes
; after the end of a short final block belong to neither file and are allowed to differ.
;
; Written by ChatGPT for Anachronaut's SplitBit

#Include services.asm

#Program
  #Base 0x5000

start:
  SETD.0 Arguments
  INIB 0xFF
  SWI osArgument
  SETD.0 Arguments
  LDA.0
  BRA usage
  CALL textSplit
  SETD.0 TextRest
  LDD.0.0
  LDA.0
  BRA usage

  SETD.0 Arguments
  SWI osFileInfo
  BNQ firstFailed
  SETD.0 FirstBlocks
  STD.3.0

  SETD.0 TextRest
  LDD.0.0
  SWI osFileInfo
  BNQ secondFailed
  SETD.0 SecondBlocks
  STD.3.0

  ; A different number of occupied blocks is immediately a different file.
  SETD.0 FirstBlocks
  LDA.0
  SETD.1 SecondBlocks
  LDB.1
  CCF
  SUB
  BNQ different
  INCD.0
  INCD.1
  LDA.0
  LDB.1
  CCF
  SUB
  BNQ different

  SETD.0 FirstBlocks
  SETD.1 RemainingBlocks
  CALL copyWord
  RSTA
  SETD.0 Index
  STA.0
  INCD.0
  STA.0

nextBlock:
  SETD.0 RemainingBlocks
  LDA.0
  INCD.0
  LDB.0
  OR
  BRQ alike

  SETD.0 Arguments
  SETD.1 FirstBlock
  CALL readAtIndex
  BNQ firstReadFailed
  SETD.0 FirstCount
  STD.3.0

  SETD.0 TextRest
  LDD.0.0
  SETD.1 SecondBlock
  CALL readAtIndex
  BNQ secondReadFailed
  SETD.0 SecondCount
  STD.3.0

  ; Equal occupied-block counts do not imply equal tails, so compare the valid byte count
  ; returned for this block as well.
  SETD.0 FirstCount
  LDA.0
  SETD.1 SecondCount
  LDB.1
  CCF
  SUB
  BNQ different
  INCD.0
  INCD.1
  LDA.0
  LDB.1
  CCF
  SUB
  BNQ different

  SETD.0 FirstCount
  SETD.1 BytesLeft
  CALL copyWord
  SETD.0 FirstBlock
  SETD.1 SecondBlock

compareByte:
  SETD.2 BytesLeft
  LDA.2
  INCD.2
  LDB.2
  OR
  BRQ blockSame
  LDA.0
  LDB.1
  CCF
  SUB
  BNQ different
  INCD.0
  INCD.1
  SETD.2 BytesLeft
  CALL takeByte
  BRI compareByte

blockSame:
  CALL stepIndex
  CALL takeBlock
  BRI nextBlock

; DP0 is the path and DP1 the destination buffer. Q and DP3 are the service answers.
readAtIndex:
  SETD.2 Index
  LDA.2
  INCD.2
  LDB.2
  SWI osFileBlock
  RET

; DP0 names the source word and DP1 the destination word.
copyWord:
  LDA.0
  STA.1
  INCD.0
  INCD.1
  LDA.0
  STA.1
  RET

stepIndex:
  SETD.0 Index
  INCD.0
  LDA.0
  INCA
  STA.0
  BNC stepDone
  DECD.0
  LDA.0
  INCA
  STA.0
stepDone:
  RET

takeBlock:
  SETD.0 RemainingBlocks
  INCD.0
  LDA.0
  BRA blockBorrow
  DECA
  STA.0
  RET
blockBorrow:
  INIA 0xFF
  STA.0
  DECD.0
  LDA.0
  DECA
  STA.0
  RET

; Decrement the big-endian sixteen-bit BytesLeft. A full block arrives as 0x0100, so an
; eight-bit counter would compare none of it and call many different files equal.
takeByte:
  INCD.2
  LDA.2
  BRA byteBorrow
  DECA
  STA.2
  RET
byteBorrow:
  INIA 0xFF
  STA.2
  DECD.2
  LDA.2
  DECA
  STA.2
  RET

alike:
  SETD.0 SameText
  SWI osPrintString
  RSTA
  SWI osExit
different:
  SETD.0 DifferentText
  SWI osPrintString
  INIA 0d1
  SWI osExit
usage:
  SETD.0 Usage
  SWI osPrintString
  INIA 0d2
  SWI osExit
firstFailed:
  SETD.0 FirstError
  SWI osPrintString
  INIA 0d2
  SWI osExit
secondFailed:
  SETD.0 SecondError
  SWI osPrintString
  INIA 0d2
  SWI osExit
firstReadFailed:
  SETD.0 FirstReadError
  SWI osPrintString
  INIA 0d2
  SWI osExit
secondReadFailed:
  SETD.0 SecondReadError
  SWI osPrintString
  INIA 0d2
  SWI osExit

#Data
  #Base 0x3000

Arguments:
  #Reserve 0d256
FirstBlocks:
  0x00 0x00
SecondBlocks:
  0x00 0x00
RemainingBlocks:
  0x00 0x00
Index:
  0x00 0x00
FirstCount:
  0x00 0x00
SecondCount:
  0x00 0x00
BytesLeft:
  0x00 0x00
FirstBlock:
  #Reserve 0d256
SecondBlock:
  #Reserve 0d256

Usage:
"compare: give me two files
"
FirstError:
"compare: cannot find the first file
"
SecondError:
"compare: cannot find the second file
"
FirstReadError:
"compare: cannot read the first file
"
SecondReadError:
"compare: cannot read the second file
"
SameText:
"the same
"
DifferentText:
"different
"

#Include text.asm