sort.rkt
#lang racket/base
(provide disk-sort)

;;; sort.scm  --  Jens Axel S√łgaard

;;; PURPOSE

; This file contains routines for sorting files larger than main memory.

;;; DEPENDENCIES

; Relies on vector-sort! from srfi-32 available from v35x.

;;; USAGE

; The file format is simply
;    <record><record><record>...

; The user provides read-record, which reads a <record>, and
; write-record that writes a record.

; The main function is:

;  (disk-sort in-file out-file block-size read-record write-record less?)

; This will take the records in in-file, sort them according to less?,
; and write them to out-file.
; The block-size indicates the number of records that can be sorted in-memory.
; It is okay to let in-file and out-file be the same.

;;; IMPLEMENTATION

(require racket/vector
         (only-in (lib "43.ss" "srfi")  vector-for-each)
         (only-in (lib "32.ss" "srfi")  vector-sort!))

(define (read-block-from-file filename position block-size read-record)
  (let ([port (open-input-file filename)])
    (begin0
      (read-block port position block-size read-record)
      (close-input-port port))))

(define (read-block port position block-size read-record)
  ; Read size records from port starting from the file-position position
  ; Return the records as a vector. If there are less than size records
  ; left in the file, the vector returned will have a length smaller than size.
  (let ([v (make-vector block-size)])
    (file-position port position)
    (let loop ([n 0])
      (cond
        [(= n block-size) v]
        [else             (let ([r (read-record port)])
                            (if (and (eof-object? r) (< n block-size))
                                ; n < size   records read
                                (set! v (vector-copy v 0 n))
                                ; store read record, and read the rest
                                (begin
                                  (vector-set! v n r)
                                  (loop (+ n 1)))))]))
    (when (for/or ([x (in-vector v)]) 
                  (eof-object? x))
      (error))
    v))


(define (merge-vectors v1 v2 less?)
  (let* ([s1 (vector-length v1)]
         [s2 (vector-length v2)]
         [v  (make-vector (+ s1 s2))])
    (let loop ([n1 0] [n2 0] [n 0])
      (cond
        [(and (= n1 s1) (= n2 s2))    v]
        [(= n1 s1)                    (begin
                                        (vector-set! v n (vector-ref v2 n2))
                                        (loop n1 (+ n2 1) (+ n 1)))]
        [(= n2 s2)                    (begin
                                        (vector-set! v n (vector-ref v1 n1))
                                        (loop (+ n1 1) n2 (+ n 1)))]
        [(less? (vector-ref v1 n1)
                (vector-ref v2 n2))   (begin
                                        (vector-set! v n (vector-ref v1 n1))
                                        (loop (+ n1 1) n2 (+ n 1)))]
        [else                         (begin
                                        (vector-set! v n (vector-ref v2 n2))
                                        (loop n1 (+ n2 1) (+ n 1)))]))))


(define (write-block vector write-record port)
  (vector-for-each (lambda (i r) (write-record r port))
                   vector))


(define (merge-blocks-in-file-once in-file out-file block-size read-record write-record less?)
  ; in-file consists of a series of blocks. Each block contains block-size records,
  ; in sorted order according to less?.
  
  ; out-file will consist of a series of blocks of size 2*block_size. Each sorted
  ; according to less?
  
  ; in-file and out-file can be the same file
  (let ([in  (open-input-file in-file)]
        [out (open-output-file out-file #:exists 'update)])
    
    (define (read-next-block)
      (read-block in (file-position in) block-size read-record))
    
    (begin0
      (let loop ([b1 (read-next-block)] 
                 [b2 (read-next-block)])
        (if (zero? (+ (vector-length b1) (vector-length b2)))
            (void)
            (begin
              (write-block (merge-vectors b1 b2 less?) write-record out)
              (loop (read-next-block) (read-next-block)))))
      (close-input-port in)
      (close-output-port out))))

(define (filename->number-of-records filename read-record)
  (let ([in (open-input-file filename)])
    (begin0
      (let loop ([n 0])
        (if (eof-object? (read-record in))
            n
            (loop (+ n 1))))
      (close-input-port in))))

(define (number-of-merges-needed size block-size)
  ; how many merges is neccessary to sort a file consisting of size records,
  ; the file consists of blocks of block-size sorted records
  (if (>= block-size size)
      0
      (+ 1 (number-of-merges-needed size (* 2 block-size)))))


(define (merge-blocks-in-file file size block-size read-record write-record less?)
  (if (zero? (number-of-merges-needed size block-size))
      (void)
      (begin
        (merge-blocks-in-file-once file file block-size read-record write-record less?)
        (merge-blocks-in-file file size (* 2 block-size) read-record write-record less?))))


(define (disk-sort in-file out-file block-size read-record write-record less?)
  ; TODO: Remember size from the first pass
  
  ; sort the records in in-filename.
  ; block-size records can be sorted in-memory
  (let ([in  (open-input-file in-file)]
        [out (open-output-file out-file #:exists 'update)])
    (define (read-next-block)
      (read-block in (file-position in) block-size read-record))
    ; Initial pass reads blocks and sort them in-memory
    (let loop ([b (read-next-block)])
      (if (zero? (vector-length b))
          (begin
            (close-input-port in)
            (close-output-port out))
          (begin
            (vector-sort! less? b)
            (write-block b write-record out)
            (loop (read-next-block)))))
    ; Finish off by repeated merging
    (let ([size (filename->number-of-records out-file read-record)])
      (merge-blocks-in-file out-file size block-size read-record write-record less?))))