; -*- Mode: Scheme; Syntax: Scheme; -*-

; This is file istruct.scm.

;;;; Interpreter data structures

; BITS-PER-BYTE
;  The compiler needs to know how many bits there are in a byte,
;  since it constructs code vectors (which are vectors of bytes).

(define bits-used-per-byte 7)  ;must be <= bits-per-byte

(define byte-limit (expt 2 bits-used-per-byte))

; Templates
;  Templates are made only by the compiler.

(define (template? obj)    ;Heuristic only, for error checking
  (and (vector? obj)
       (>=& (vector-length obj) 2)
       (code-vector? (template-code obj))))

(define (template-code tem) (vector-ref tem 0))
(define (template-name tem) (vector-ref tem 1))


; Continuations
;  Continuations are made only by the interpreter.

(define (continuation-pc c)       (vector-ref c 0))
(define (continuation-template c) (vector-ref c 1))
(define (continuation-env c)	  (vector-ref c 2))
(define (continuation-cont c)	  (vector-ref c 3))


; Interpreter state
;  Interpreter states are made only by the interpreter.

(define (istate-cont i)  (vector-ref i 0))
(define (istate-nargs i) (vector-ref i 1))
(define (istate-val i)	 (vector-ref i 2))
(define (istate-arg2 i)	 (vector-ref i 3))
(define (istate-arg3 i)	 (vector-ref i 4))
(define (istate-ei i)	 (vector-ref i 5))


; Cf. struct.scm:
;
; The hash function was tested on 607 symbols from the
; scheme-48 sources.  Whether or not the symbol table size (modulus)
; was prime or not was found not to make much difference; in fact,
; moduli of 256 and 512 worked out pretty well.  The standard
; deviation for the length of the buckets was as follows:
;      199           1.744
;      256           1.695
;      509           1.175
;      512           1.202
;      1021          0.828
; Since taking a remainder mod 512 is much faster than taking one mod
; 509, 512 is the choice here for the table size.

(define log-table-size 9)
(define table-size (expt 2 log-table-size))

(define (make-hash-table)
  (let ((table (make-vector table-size)))
    (vector-fill! table null)
    table))

(define make-global-environment make-hash-table)

(define make-symbol-table make-hash-table)