%include "CONSTANTS.h"

   RNG_A equ 1103515245
   RNG_C equ 12345
   RNG_M equ 4000000x

section .data
   RNG_SEED: dq 1

   randTestFMT: db `call %d = %d\n`,0
   ;randRangeFMT: db `1D%d = %d\n`,0
   randInRangeFMT: db `%d\n`,0
section .bss

section .text

global main
main:

main_done:

    ; seed the RNG
    mov rdi, 0
    mov rax, 100
    syscall

    mov [RNG_SEED], rax

    ;mov rdi, 20
    ;call RAND_TEST

    mov rdi, 10
    mov rsi, 1
    mov rdx, 20
    call SIMPLE_RANGE_TEST

    jmp Exit

SIMPLE_RANGE_TEST:
  ; this function will generage a number of values in a gvien range
  ; rdi will hold the number (n)  of elements to generate
  ; rsi will hold the low value for the range
  ; rdx will hold the high value for the range

  ; r12 will be the low value
  ; r13 will be used for the range
  ; r14 will be the lcv

  ; preamble
  push rbp
  mov rbp, rsp

  push r12
  push r13
  push r14

  push rdi  ; save n

  ; r12 = low
  mov r12, rsi

  ; range = high - low +1
  mov r13, rdx
  sub r13, rsi
  inc r13

  ; i = 0
  mov r14, 0

RANGE_TEST_START:
  ; while i < n
  cmp r14, [rsp]
  je  RANGE_TEST_DONE

  ;    cout << RandInRange(range, low) 
  mov rdi, r13
  mov rsi, r12
  call RAND_IN_RANGE

  mov rdi, randInRangeFMT
  mov rsi, rax
  call CallPrintf

  ;    i++
  inc r14
  jmp RANGE_TEST_START

RANGE_TEST_DONE:
  ; epilogue

  pop rdi

  pop r14
  pop r13
  pop r12

  pop rbp
  ret


RAND_IN_RANGE:
  ; this function will generate a random value in a range.
  ; rdi will hold the range size
  ; rsi will hold the min vlaue
  ; return the value in rax

  ;   rax = rand() % rdi + rsi

  ; we will save rsi and rdi just to make sure we can.

  ;  prefix
  push rbp
  mov  rbp, rsp

  push rsi  ; the min value
  push rdi  ; the range size

  call RAND
  ; rax will no hold the random number

  mov rdx, 0
  ; rax % range
  pop r8   ; load the value of range size
  div r8
 
  ; rax + min
  pop rax  ; load the minimum value
  add rax, rdx

  ; epilogue

  pop rbp
  ret

RAND_TEST:
  ; this function will call the rand number generator a gvien number of times

  ; input: in rdi will be the number of times to call rand
  ; output: none

  ; not needed, but I want to use R12 as my LCV so I need to save it.
  ; not needed, but I want to use R13 as the limit 

  ; prefix
  push rbp
  mov rbp, rsp

  push r12
  push r13

  ; rdi might be wiped out in a function call, so move this to r13
  mov r13, rdi

  ; i = 0
  mov r12, 0

  ; while i < n
TOP_OF_RAND_TEST_LOOP:

  cmp r12, r13
  je OUT_OF_RAND_TEST_LOOP


  call RAND

  mov rdx, rax
  mov rsi, r12
  mov rdi, randTestFMT
  call CallPrintf

  ;  cout << i << " " << rand() << endl
  ;  ++i
  inc r12
  jmp  TOP_OF_RAND_TEST_LOOP

OUT_OF_RAND_TEST_LOOP:

  ; epilogue 

  pop r13
  pop r12

  pop rbp
  ret

RAND:
  ; no input parameters
  ; the result will be returned in rax

  ; this is a leaf routine, no need to build an AR

  ; load the previous seed
  mov rax, qword [RNG_SEED]

  ; multiply this by A
  mov rdx, 0
  mov r10, RNG_A
  mul r10 

  ; add c to it
  add rax, RNG_C

  ; mod by m
  mov rdx, 0
  mov r10, RNG_M
  div r10 

  ; save the in the global seed
  mov rax, rdx
  mov qword[RNG_SEED], rax

  ; return it in rax
  
  ret