]> cvs.zerfleddert.de Git - proxmark3-svn/blobdiff - armsrc/util.c
added changelog
[proxmark3-svn] / armsrc / util.c
index ae2e724738d64f470c966a58dd34322dcaf3f64b..a0fa50d3a3bc23c68ba7d41d44db749297adf63e 100644 (file)
@@ -1,78 +1,61 @@
 //-----------------------------------------------------------------------------
 //-----------------------------------------------------------------------------
-// Utility functions used in many places, not specific to any piece of code.
 // Jonathan Westhues, Sept 2005
 // Jonathan Westhues, Sept 2005
+//
+// This code is licensed to you under the terms of the GNU GPL, version 2 or,
+// at your option, any later version. See the LICENSE.txt file for the text of
+// the license.
+//-----------------------------------------------------------------------------
+// Utility functions used in many places, not specific to any piece of code.
 //-----------------------------------------------------------------------------
 //-----------------------------------------------------------------------------
+
 #include "proxmark3.h"
 #include "proxmark3.h"
+#include "util.h"
+#include "string.h"
 #include "apps.h"
 #include "apps.h"
-
-void *memcpy(void *dest, const void *src, int len)
-{
-       BYTE *d = dest;
-       const BYTE *s = src;
-       while((len--) > 0) {
-               *d = *s;
-               d++;
-               s++;
-       }
-       return dest;
-}
-
-void *memset(void *dest, int c, int len)
-{
-       BYTE *d = dest;
-       while((len--) > 0) {
-               *d = c;
-               d++;
-       }
-       return dest;
+#include "BigBuf.h"
+
+
+
+void print_result(char *name, uint8_t *buf, size_t len) {
+   uint8_t *p = buf;
+
+   if ( len % 16 == 0 ) {
+          for(; p-buf < len; p += 16)
+       Dbprintf("[%s:%d/%d] %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x",
+                               name,
+                               p-buf,
+                               len,
+                               p[0], p[1], p[2], p[3], p[4], p[5], p[6], p[7],p[8], p[9], p[10], p[11], p[12], p[13], p[14], p[15]
+          );
+   }
+   else {
+   for(; p-buf < len; p += 8)
+       Dbprintf("[%s:%d/%d] %02x %02x %02x %02x %02x %02x %02x %02x", name, p-buf, len, p[0], p[1], p[2], p[3], p[4], p[5], p[6], p[7]);
+   }
 }
 
 }
 
-int memcmp(const void *av, const void *bv, int len)
-{
-       const BYTE *a = av;
-       const BYTE *b = bv;
-
-       while((len--) > 0) {
-               if(*a != *b) {
-                       return *a - *b;
-               }
-               a++;
-               b++;
-       }
-       return 0;
+size_t nbytes(size_t nbits) {
+       return (nbits >> 3)+((nbits % 8) > 0);
 }
 
 }
 
-int strlen(char *str)
-{
-       int l = 0;
-       while(*str) {
-               l++;
-               str++;
+uint32_t SwapBits(uint32_t value, int nrbits) {
+       int i;
+       uint32_t newvalue = 0;
+       for(i = 0; i < nrbits; i++) {
+               newvalue ^= ((value >> i) & 1) << (nrbits - 1 - i);
        }
        }
-       return l;
+       return newvalue;
 }
 
 }
 
-char* strncat(char *dest, const char *src, unsigned int n)
-{
-       unsigned int dest_len = strlen(dest);
-       unsigned int i;
-
-       for (i = 0 ; i < n && src[i] != '\0' ; i++)
-               dest[dest_len + i] = src[i];
-       dest[dest_len + i] = '\0';
-
-       return dest;
-}
-
-void num_to_bytes(uint64_t n, size_t len, byte_t* dest)
+void num_to_bytes(uint64_t n, size_t len, uint8_t* dest)
 {
        while (len--) {
 {
        while (len--) {
-               dest[len] = (byte_t) n;
+               dest[len] = (uint8_t) n;
                n >>= 8;
        }
 }
 
                n >>= 8;
        }
 }
 
-uint64_t bytes_to_num(byte_t* src, size_t len)
+uint64_t bytes_to_num(uint8_t* src, size_t len)
 {
        uint64_t num = 0;
        while (len--)
 {
        uint64_t num = 0;
        while (len--)
@@ -83,6 +66,26 @@ uint64_t bytes_to_num(byte_t* src, size_t len)
        return num;
 }
 
        return num;
 }
 
+// RotateLeft - Ultralight, Desfire
+void rol(uint8_t *data, const size_t len){
+    uint8_t first = data[0];
+    for (size_t i = 0; i < len-1; i++) {
+        data[i] = data[i+1];
+    }
+    data[len-1] = first;
+}
+void lsl (uint8_t *data, size_t len) {
+    for (size_t n = 0; n < len - 1; n++) {
+        data[n] = (data[n] << 1) | (data[n+1] >> 7);
+    }
+    data[len - 1] <<= 1;
+}
+
+int32_t le24toh (uint8_t data[3])
+{
+    return (data[2] << 16) | (data[1] << 8) | data[0];
+}
+
 void LEDsoff()
 {
        LED_A_OFF();
 void LEDsoff()
 {
        LED_A_OFF();
@@ -139,12 +142,12 @@ int BUTTON_CLICKED(int ms)
        AT91C_BASE_PWMC_CH0->PWMC_CDTYR = 0;
        AT91C_BASE_PWMC_CH0->PWMC_CPRDR = 0xffff;
 
        AT91C_BASE_PWMC_CH0->PWMC_CDTYR = 0;
        AT91C_BASE_PWMC_CH0->PWMC_CPRDR = 0xffff;
 
-       WORD start = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
+       uint16_t start = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
 
        int letoff = 0;
        for(;;)
        {
 
        int letoff = 0;
        for(;;)
        {
-               WORD now = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
+               uint16_t now = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
 
                // We haven't let off the button yet
                if (!letoff)
 
                // We haven't let off the button yet
                if (!letoff)
@@ -162,7 +165,7 @@ int BUTTON_CLICKED(int ms)
                        // Still haven't let it off
                        else
                                // Have we held down a full second?
                        // Still haven't let it off
                        else
                                // Have we held down a full second?
-                               if (now == (WORD)(start + ticks))
+                               if (now == (uint16_t)(start + ticks))
                                        return BUTTON_HOLD;
                }
 
                                        return BUTTON_HOLD;
                }
 
@@ -174,7 +177,7 @@ int BUTTON_CLICKED(int ms)
 
                        // Have we ran out of time to double click?
                        else
 
                        // Have we ran out of time to double click?
                        else
-                               if (now == (WORD)(start + ticks))
+                               if (now == (uint16_t)(start + ticks))
                                        // At least we did a single click
                                        return BUTTON_SINGLE_CLICK;
 
                                        // At least we did a single click
                                        return BUTTON_SINGLE_CLICK;
 
@@ -202,11 +205,11 @@ int BUTTON_HELD(int ms)
        AT91C_BASE_PWMC_CH0->PWMC_CDTYR = 0;
        AT91C_BASE_PWMC_CH0->PWMC_CPRDR = 0xffff;
 
        AT91C_BASE_PWMC_CH0->PWMC_CDTYR = 0;
        AT91C_BASE_PWMC_CH0->PWMC_CPRDR = 0xffff;
 
-       WORD start = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
+       uint16_t start = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
 
        for(;;)
        {
 
        for(;;)
        {
-               WORD now = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
+               uint16_t now = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
 
                // As soon as our button let go, we didn't hold long enough
                if (!BUTTON_PRESS())
 
                // As soon as our button let go, we didn't hold long enough
                if (!BUTTON_PRESS())
@@ -214,7 +217,7 @@ int BUTTON_HELD(int ms)
 
                // Have we waited the full second?
                else
 
                // Have we waited the full second?
                else
-                       if (now == (WORD)(start + ticks))
+                       if (now == (uint16_t)(start + ticks))
                                return BUTTON_HOLD;
 
                WDT_HIT();
                                return BUTTON_HOLD;
 
                WDT_HIT();
@@ -237,11 +240,11 @@ void SpinDelayUs(int us)
        AT91C_BASE_PWMC_CH0->PWMC_CDTYR = 0;
        AT91C_BASE_PWMC_CH0->PWMC_CPRDR = 0xffff;
 
        AT91C_BASE_PWMC_CH0->PWMC_CDTYR = 0;
        AT91C_BASE_PWMC_CH0->PWMC_CPRDR = 0xffff;
 
-       WORD start = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
+       uint16_t start = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
 
        for(;;) {
 
        for(;;) {
-               WORD now = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
-               if (now == (WORD)(start + ticks))
+               uint16_t now = AT91C_BASE_PWMC_CH0->PWMC_CCNTR;
+               if (now == (uint16_t)(start + ticks))
                        return;
 
                WDT_HIT();
                        return;
 
                WDT_HIT();
@@ -263,27 +266,250 @@ void FormatVersionInformation(char *dst, int len, const char *prefix, void *vers
 {
        struct version_information *v = (struct version_information*)version_information;
        dst[0] = 0;
 {
        struct version_information *v = (struct version_information*)version_information;
        dst[0] = 0;
-       strncat(dst, prefix, len);
+       strncat(dst, prefix, len-1);
        if(v->magic != VERSION_INFORMATION_MAGIC) {
        if(v->magic != VERSION_INFORMATION_MAGIC) {
-               strncat(dst, "Missing/Invalid version information", len);
+               strncat(dst, "Missing/Invalid version information\n", len - strlen(dst) - 1);
                return;
        }
        if(v->versionversion != 1) {
                return;
        }
        if(v->versionversion != 1) {
-               strncat(dst, "Version information not understood", len);
+               strncat(dst, "Version information not understood\n", len - strlen(dst) - 1);
                return;
        }
        if(!v->present) {
                return;
        }
        if(!v->present) {
-               strncat(dst, "Version information not available", len);
+               strncat(dst, "Version information not available\n", len - strlen(dst) - 1);
                return;
        }
 
                return;
        }
 
-       strncat(dst, v->svnversion, len);
+       strncat(dst, v->gitversion, len - strlen(dst) - 1);
        if(v->clean == 0) {
        if(v->clean == 0) {
-               strncat(dst, "-unclean", len);
+               strncat(dst, "-unclean", len - strlen(dst) - 1);
        } else if(v->clean == 2) {
        } else if(v->clean == 2) {
-               strncat(dst, "-suspect", len);
+               strncat(dst, "-suspect", len - strlen(dst) - 1);
        }
 
        }
 
-       strncat(dst, " ", len);
-       strncat(dst, v->buildtime, len);
+       strncat(dst, " ", len - strlen(dst) - 1);
+       strncat(dst, v->buildtime, len - strlen(dst) - 1);
+       strncat(dst, "\n", len - strlen(dst) - 1);
+}
+
+//  -------------------------------------------------------------------------
+//  timer lib
+//  -------------------------------------------------------------------------
+//  test procedure:
+//
+//     ti = GetTickCount();
+//     SpinDelay(1000);
+//     ti = GetTickCount() - ti;
+//     Dbprintf("timer(1s): %d t=%d", ti, GetTickCount());
+
+void StartTickCount()
+{
+       // This timer is based on the slow clock. The slow clock frequency is between 22kHz and 40kHz.
+       // We can determine the actual slow clock frequency by looking at the Main Clock Frequency Register.
+    uint16_t mainf = AT91C_BASE_PMC->PMC_MCFR & 0xffff;                // = 16 * main clock frequency (16MHz) / slow clock frequency
+       // set RealTimeCounter divider to count at 1kHz:
+       AT91C_BASE_RTTC->RTTC_RTMR = AT91C_RTTC_RTTRST | ((256000 + (mainf/2)) / mainf);
+       // note: worst case precision is approx 2.5%
+}
+
+/*
+* Get the current count.
+*/
+uint32_t RAMFUNC GetTickCount(){
+       return AT91C_BASE_RTTC->RTTC_RTVR;// was * 2;
 }
 }
+
+//  -------------------------------------------------------------------------
+//  microseconds timer 
+//  -------------------------------------------------------------------------
+void StartCountUS()
+{
+       AT91C_BASE_PMC->PMC_PCER |= (0x1 << 12) | (0x1 << 13) | (0x1 << 14);
+//     AT91C_BASE_TCB->TCB_BMR = AT91C_TCB_TC1XC1S_TIOA0;
+       AT91C_BASE_TCB->TCB_BMR = AT91C_TCB_TC0XC0S_NONE | AT91C_TCB_TC1XC1S_TIOA0 | AT91C_TCB_TC2XC2S_NONE;
+
+       // fast clock
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKDIS; // timer disable
+       AT91C_BASE_TC0->TC_CMR = AT91C_TC_CLKS_TIMER_DIV3_CLOCK | // MCK(48MHz)/32 -- tick=1.5mks
+                                                                                                               AT91C_TC_WAVE | AT91C_TC_WAVESEL_UP_AUTO | AT91C_TC_ACPA_CLEAR |
+                                                                                                               AT91C_TC_ACPC_SET | AT91C_TC_ASWTRG_SET;
+       AT91C_BASE_TC0->TC_RA = 1;
+       AT91C_BASE_TC0->TC_RC = 0xBFFF + 1; // 0xC000
+       
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKDIS; // timer disable  
+       AT91C_BASE_TC1->TC_CMR = AT91C_TC_CLKS_XC1; // from timer 0
+       
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKEN;
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKEN;
+       AT91C_BASE_TCB->TCB_BCR = 1;
+       }
+
+uint32_t RAMFUNC GetCountUS(){
+       return (AT91C_BASE_TC1->TC_CV * 0x8000) + ((AT91C_BASE_TC0->TC_CV * 2) / 3); //was  /15) * 10);
+}
+
+static uint32_t GlobalUsCounter = 0;
+
+uint32_t RAMFUNC GetDeltaCountUS(){
+       uint32_t g_cnt = GetCountUS();
+       uint32_t g_res = g_cnt - GlobalUsCounter;
+       GlobalUsCounter = g_cnt;
+       return g_res;
+}
+
+
+//  -------------------------------------------------------------------------
+//  Timer for iso14443 commands. Uses ssp_clk from FPGA 
+//  -------------------------------------------------------------------------
+void StartCountSspClk()
+{
+       AT91C_BASE_PMC->PMC_PCER = (1 << AT91C_ID_TC0) | (1 << AT91C_ID_TC1) | (1 << AT91C_ID_TC2);  // Enable Clock to all timers
+       AT91C_BASE_TCB->TCB_BMR = AT91C_TCB_TC0XC0S_TIOA1               // XC0 Clock = TIOA1
+                                                       | AT91C_TCB_TC1XC1S_NONE                // XC1 Clock = none
+                                                       | AT91C_TCB_TC2XC2S_TIOA0;              // XC2 Clock = TIOA0
+
+       // configure TC1 to create a short pulse on TIOA1 when a rising edge on TIOB1 (= ssp_clk from FPGA) occurs:
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKDIS;                               // disable TC1
+       AT91C_BASE_TC1->TC_CMR = AT91C_TC_CLKS_TIMER_DIV1_CLOCK // TC1 Clock = MCK(48MHz)/2 = 24MHz
+                                                       | AT91C_TC_CPCSTOP                              // Stop clock on RC compare
+                                                       | AT91C_TC_EEVTEDG_RISING               // Trigger on rising edge of Event
+                                                       | AT91C_TC_EEVT_TIOB                    // Event-Source: TIOB1 (= ssp_clk from FPGA = 13,56MHz/16)
+                                                       | AT91C_TC_ENETRG                               // Enable external trigger event
+                                                       | AT91C_TC_WAVESEL_UP                   // Upmode without automatic trigger on RC compare
+                                                       | AT91C_TC_WAVE                                 // Waveform Mode
+                                                       | AT91C_TC_AEEVT_SET                    // Set TIOA1 on external event
+                                                       | AT91C_TC_ACPC_CLEAR;                  // Clear TIOA1 on RC Compare
+       AT91C_BASE_TC1->TC_RC = 0x04;                                                   // RC Compare value = 0x04
+
+       // use TC0 to count TIOA1 pulses
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKDIS;                               // disable TC0
+       AT91C_BASE_TC0->TC_CMR = AT91C_TC_CLKS_XC0                              // TC0 clock = XC0 clock = TIOA1
+                                                       | AT91C_TC_WAVE                                 // Waveform Mode
+                                                       | AT91C_TC_WAVESEL_UP                   // just count
+                                                       | AT91C_TC_ACPA_CLEAR                   // Clear TIOA0 on RA Compare
+                                                       | AT91C_TC_ACPC_SET;                    // Set TIOA0 on RC Compare
+       AT91C_BASE_TC0->TC_RA = 1;                                                              // RA Compare value = 1; pulse width to TC2
+       AT91C_BASE_TC0->TC_RC = 0;                                                              // RC Compare value = 0; increment TC2 on overflow
+
+       // use TC2 to count TIOA0 pulses (giving us a 32bit counter (TC0/TC2) clocked by ssp_clk)
+       AT91C_BASE_TC2->TC_CCR = AT91C_TC_CLKDIS;                               // disable TC2  
+       AT91C_BASE_TC2->TC_CMR = AT91C_TC_CLKS_XC2                              // TC2 clock = XC2 clock = TIOA0
+                                                       | AT91C_TC_WAVE                                 // Waveform Mode
+                                                       | AT91C_TC_WAVESEL_UP;                  // just count
+       
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKEN;                                // enable TC0
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKEN;                                // enable TC1
+       AT91C_BASE_TC2->TC_CCR = AT91C_TC_CLKEN;                                // enable TC2
+
+       //
+       // synchronize the counter with the ssp_frame signal. Note: FPGA must be in any iso14446 mode, otherwise the frame signal would not be present 
+       //
+       while(!(AT91C_BASE_PIOA->PIO_PDSR & GPIO_SSC_FRAME));   // wait for ssp_frame to go high (start of frame)
+       while(AT91C_BASE_PIOA->PIO_PDSR & GPIO_SSC_FRAME);              // wait for ssp_frame to be low
+       while(!(AT91C_BASE_PIOA->PIO_PDSR & GPIO_SSC_CLK));     // wait for ssp_clk to go high
+       // note: up to now two ssp_clk rising edges have passed since the rising edge of ssp_frame
+       // it is now safe to assert a sync signal. This sets all timers to 0 on next active clock edge
+       AT91C_BASE_TCB->TCB_BCR = 1;                                                    // assert Sync (set all timers to 0 on next active clock edge)
+       // at the next (3rd) ssp_clk rising edge, TC1 will be reset (and not generate a clock signal to TC0)
+       // at the next (4th) ssp_clk rising edge, TC0 (the low word of our counter) will be reset. From now on,
+       // whenever the last three bits of our counter go 0, we can be sure to be in the middle of a frame transfer.
+       // (just started with the transfer of the 4th Bit).
+       // The high word of the counter (TC2) will not reset until the low word (TC0) overflows. Therefore need to wait quite some time before
+       // we can use the counter.
+       while (AT91C_BASE_TC0->TC_CV < 0xFFF0);
+}
+void ResetSspClk(void) {
+       //enable clock of timer and software trigger
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKEN | AT91C_TC_SWTRG;
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKEN | AT91C_TC_SWTRG;
+       AT91C_BASE_TC2->TC_CCR = AT91C_TC_CLKEN | AT91C_TC_SWTRG;
+       while (AT91C_BASE_TC2->TC_CV > 0);
+}
+uint32_t RAMFUNC GetCountSspClk(){
+       uint32_t tmp_count;
+       tmp_count = (AT91C_BASE_TC2->TC_CV << 16) | AT91C_BASE_TC0->TC_CV;
+       if ((tmp_count & 0x0000ffff) == 0) { //small chance that we may have missed an increment in TC2
+               return (AT91C_BASE_TC2->TC_CV << 16);
+       } 
+       else {
+               return tmp_count;
+       }
+}
+
+//  -------------------------------------------------------------------------
+//  Timer for bitbanging,  or LF stuff when you need a very precis timer
+//  1us = 1.5ticks
+//  -------------------------------------------------------------------------
+void StartTicks(void){
+       //initialization of the timer
+       // tc1 is higher 0xFFFF0000
+       // tc0 is lower 0x0000FFFF
+       AT91C_BASE_PMC->PMC_PCER |= (1 << AT91C_ID_TC0) | (1 << AT91C_ID_TC1);
+       AT91C_BASE_TCB->TCB_BMR = AT91C_TCB_TC0XC0S_NONE | AT91C_TCB_TC1XC1S_TIOA0 | AT91C_TCB_TC2XC2S_NONE;
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKDIS;
+       AT91C_BASE_TC0->TC_CMR = AT91C_TC_CLKS_TIMER_DIV3_CLOCK | // MCK(48MHz) / 32 
+                                                               AT91C_TC_WAVE | AT91C_TC_WAVESEL_UP_AUTO | AT91C_TC_ACPA_CLEAR |
+                                                               AT91C_TC_ACPC_SET | AT91C_TC_ASWTRG_SET;
+       AT91C_BASE_TC0->TC_RA = 1;
+       AT91C_BASE_TC0->TC_RC = 0; 
+
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKDIS;       // timer disable  
+       AT91C_BASE_TC1->TC_CMR = AT91C_TC_CLKS_XC1; // from TC0
+       
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKEN | AT91C_TC_SWTRG;
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKEN | AT91C_TC_SWTRG;
+       AT91C_BASE_TCB->TCB_BCR = 1;
+       
+       // wait until timer becomes zero.
+       while (AT91C_BASE_TC1->TC_CV > 0);
+}
+
+// Wait - Spindelay in ticks.
+// if called with a high number, this will trigger the WDT...
+void WaitTicks(uint32_t ticks){
+       if ( ticks == 0 ) return;
+       ticks += GET_TICKS;     
+       while (GET_TICKS < ticks);
+}
+// Wait / Spindelay in us (microseconds) 
+// 1us = 1.5ticks.
+void WaitUS(uint16_t us){
+       if ( us == 0 ) return;
+       WaitTicks(  (uint32_t)(us * 1.5) );
+}
+void WaitMS(uint16_t ms){
+       if (ms == 0) return;
+       WaitTicks( (uint32_t)(ms * 1500) );
+}
+// Starts Clock and waits until its reset
+void ResetTicks(void){
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKEN | AT91C_TC_SWTRG;
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKEN | AT91C_TC_SWTRG;
+       while (AT91C_BASE_TC1->TC_CV > 0);
+}
+void ResetTimer(AT91PS_TC timer){
+       timer->TC_CCR = AT91C_TC_CLKEN | AT91C_TC_SWTRG;
+       while(timer->TC_CV > 0) ;
+}
+// stop clock
+void StopTicks(void){
+       AT91C_BASE_TC0->TC_CCR = AT91C_TC_CLKDIS;
+       AT91C_BASE_TC1->TC_CCR = AT91C_TC_CLKDIS;       
+}
+
+static uint64_t next_random = 1;
+
+/* Generates a (non-cryptographically secure) 32-bit random number.
+ *
+ * We don't have an implementation of the "rand" function or a clock to seed it
+ * with, so we just call GetTickCount the first time to seed ourselves.
+ */
+uint32_t prand() {
+       if (next_random == 1) {
+               next_random = GetTickCount();
+       }
+
+       next_random = next_random * 6364136223846793005 + 1;
+       return (uint32_t)(next_random >> 32) % 0xffffffff;
+}
+
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