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/*
* EEZ Generic Firmware
* Copyright (C) 2018-present, Envox d.o.o.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include <eez/core/util.h>
#define _USE_MATH_DEFINES
#include <math.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#if defined(EEZ_PLATFORM_STM32)
#include <crc.h>
#endif
namespace eez {
float remap(float x, float x1, float y1, float x2, float y2) {
return y1 + (x - x1) * (y2 - y1) / (x2 - x1);
}
float remapQuad(float x, float x1, float y1, float x2, float y2) {
float t = remap(x, x1, 0, x2, 1);
t = t * t;
x = remap(t, 0, x1, 1, x2);
return remap(x, x1, y1, x2, y2);
}
float remapOutQuad(float x, float x1, float y1, float x2, float y2) {
float t = remap(x, x1, 0, x2, 1);
t = t * (2 - t);
x = remap(t, 0, x1, 1, x2);
return remap(x, x1, y1, x2, y2);
}
float remapInOutQuad(float x, float x1, float y1, float x2, float y2) {
float t = remap(x, x1, 0, x2, 1);
t = t < .5 ? 2 * t*t : -1 + (4 - 2 * t)*t;
x = remap(t, 0, x1, 1, x2);
return remap(x, x1, y1, x2, y2);
}
float remapCubic(float x, float x1, float y1, float x2, float y2) {
float t = remap(x, x1, 0, x2, 1);
t = t * t * t;
x = remap(t, 0, x1, 1, x2);
return remap(x, x1, y1, x2, y2);
}
float remapOutCubic(float x, float x1, float y1, float x2, float y2) {
float t = remap(x, x1, 0, x2, 1);
t = t - 1;
t = 1 + t * t * t;
x = remap(t, 0, x1, 1, x2);
return remap(x, x1, y1, x2, y2);
}
float remapExp(float x, float x1, float y1, float x2, float y2) {
float t = remap(x, x1, 0, x2, 1);
t = t == 0 ? 0 : float(pow(2, 10 * (t - 1)));
x = remap(t, 0, x1, 1, x2);
return remap(x, x1, y1, x2, y2);
}
float remapOutExp(float x, float x1, float y1, float x2, float y2) {
float t = remap(x, x1, 0, x2, 1);
t = t == 1 ? 1 : float(1 - pow(2, -10 * t));
x = remap(t, 0, x1, 1, x2);
return remap(x, x1, y1, x2, y2);
}
float clamp(float x, float min, float max) {
if (x < min) {
return min;
}
if (x > max) {
return max;
}
return x;
}
void stringCopy(char *dst, size_t maxStrLength, const char *src) {
strncpy(dst, src, maxStrLength);
dst[maxStrLength - 1] = 0;
}
void stringCopyLength(char *dst, size_t maxStrLength, const char *src, size_t length) {
size_t n = MIN(length, maxStrLength);
strncpy(dst, src, n);
dst[n] = 0;
}
void stringAppendString(char *str, size_t maxStrLength, const char *value) {
int n = maxStrLength - strlen(str) - 1;
if (n >= 0) {
strncat(str, value, n);
}
}
void stringAppendStringLength(char *str, size_t maxStrLength, const char *value, size_t length) {
int n = MIN(maxStrLength - strlen(str) - 1, length);
if (n >= 0) {
strncat(str, value, n);
}
}
void stringAppendInt(char *str, size_t maxStrLength, int value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%d", value);
}
void stringAppendUInt32(char *str, size_t maxStrLength, uint32_t value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%lu", (unsigned long)value);
}
void stringAppendInt64(char *str, size_t maxStrLength, int64_t value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%jd", value);
}
void stringAppendUInt64(char *str, size_t maxStrLength, uint64_t value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%ju", value);
}
void stringAppendFloat(char *str, size_t maxStrLength, float value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%g", value);
}
void stringAppendFloat(char *str, size_t maxStrLength, float value, int numDecimalPlaces) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%.*f", numDecimalPlaces, value);
}
void stringAppendDouble(char *str, size_t maxStrLength, double value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%g", value);
}
void stringAppendDouble(char *str, size_t maxStrLength, double value, int numDecimalPlaces) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%.*f", numDecimalPlaces, value);
}
void stringAppendVoltage(char *str, size_t maxStrLength, float value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%g V", value);
}
void stringAppendCurrent(char *str, size_t maxStrLength, float value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%g A", value);
}
void stringAppendPower(char *str, size_t maxStrLength, float value) {
auto n = strlen(str);
snprintf(str + n, maxStrLength - n, "%g W", value);
}
void stringAppendDuration(char *str, size_t maxStrLength, float value) {
auto n = strlen(str);
if (value > 0.1) {
snprintf(str + n, maxStrLength - n, "%g s", value);
} else {
snprintf(str + n, maxStrLength - n, "%g ms", value * 1000);
}
}
void stringAppendLoad(char *str, size_t maxStrLength, float value) {
auto n = strlen(str);
if (value < 1000) {
snprintf(str + n, maxStrLength - n, "%g ohm", value);
} else if (value < 1000000) {
snprintf(str + n, maxStrLength - n, "%g Kohm", value / 1000);
} else {
snprintf(str + n, maxStrLength - n, "%g Mohm", value / 1000000);
}
}
#if defined(EEZ_PLATFORM_STM32)
uint32_t crc32(const uint8_t *mem_block, size_t block_size) {
return HAL_CRC_Calculate(&hcrc, (uint32_t *)mem_block, block_size);
}
#else
/*
From http://www.hackersdelight.org/hdcodetxt/crc.c.txt:
This is the basic CRC-32 calculation with some optimization but no
table lookup. The the byte reversal is avoided by shifting the crc reg
right instead of left and by using a reversed 32-bit word to represent
the polynomial.
When compiled to Cyclops with GCC, this function executes in 8 + 72n
instructions, where n is the number of bytes in the input message. It
should be doable in 4 + 61n instructions.
If the inner loop is strung out (approx. 5*8 = 40 instructions),
it would take about 6 + 46n instructions.
*/
uint32_t crc32(const uint8_t *mem_block, size_t block_size) {
uint32_t crc = 0xFFFFFFFF;
for (size_t i = 0; i < block_size; ++i) {
uint32_t byte = mem_block[i]; // Get next byte.
crc = crc ^ byte;
for (int j = 0; j < 8; ++j) { // Do eight times.
uint32_t mask = -((int32_t)crc & 1);
crc = (crc >> 1) ^ (0xEDB88320 & mask);
}
}
return ~crc;
}
#endif
uint8_t toBCD(uint8_t bin) {
return ((bin / 10) << 4) | (bin % 10);
}
uint8_t fromBCD(uint8_t bcd) {
return ((bcd >> 4) & 0xF) * 10 + (bcd & 0xF);
}
float roundPrec(float a, float prec) {
float r = 1 / prec;
return roundf(a * r) / r;
}
float floorPrec(float a, float prec) {
float r = 1 / prec;
return floorf(a * r) / r;
}
float ceilPrec(float a, float prec) {
float r = 1 / prec;
return ceilf(a * r) / r;
}
bool isNaN(float x) {
return x != x;
}
bool isNaN(double x) {
return x != x;
}
bool isDigit(char ch) {
return ch >= '0' && ch <= '9';
}
bool isHexDigit(char ch) {
return (ch >= '0' && ch <= '9') || (ch >= 'a' && ch <= 'f') || (ch >= 'A' && ch <= 'F');
}
bool isUperCaseLetter(char ch) {
return ch >= 'A' && ch <= 'Z';
}
char toHexDigit(int num) {
if (num >= 0 && num <= 9) {
return '0' + num;
} else {
return 'A' + (num - 10);
}
}
int fromHexDigit(char ch) {
if (ch >= '0' && ch <= '9') {
return ch - '0';
}
if (ch >= 'a' && ch <= 'f') {
return 10 + (ch - 'a');
}
return 10 + (ch - 'A');
}
bool pointInsideRect(int xPoint, int yPoint, int xRect, int yRect, int wRect, int hRect) {
return xPoint >= xRect && xPoint < xRect + wRect && yPoint >= yRect && yPoint < yRect + hRect;
}
void getParentDir(const char *path, char *parentDirPath) {
int lastPathSeparatorIndex;
for (lastPathSeparatorIndex = strlen(path) - 1;
lastPathSeparatorIndex >= 0 && path[lastPathSeparatorIndex] != PATH_SEPARATOR[0];
--lastPathSeparatorIndex)
;
int i;
for (i = 0; i < lastPathSeparatorIndex; ++i) {
parentDirPath[i] = path[i];
}
parentDirPath[i] = 0;
}
bool parseMacAddress(const char *macAddressStr, size_t macAddressStrLength, uint8_t *macAddress) {
int state = 0;
int a = 0;
int i = 0;
uint8_t resultMacAddress[6];
const char *end = macAddressStr + macAddressStrLength;
for (const char *p = macAddressStr; p < end; ++p) {
if (state == 0) {
if (*p == '-' || *p == ' ') {
continue;
} else if (isHexDigit(*p)) {
a = fromHexDigit(*p);
state = 1;
} else {
return false;
}
} else if (state == 1) {
if (isHexDigit(*p)) {
if (i < 6) {
resultMacAddress[i++] = (a << 4) | fromHexDigit(*p);
state = 0;
} else {
return false;
}
} else {
return false;
}
}
}
if (state != 0 || i != 6) {
return false;
}
memcpy(macAddress, resultMacAddress, 6);
return true;
}
bool parseIpAddress(const char *ipAddressStr, size_t ipAddressStrLength, uint32_t &ipAddress) {
const char *p = ipAddressStr;
const char *q = ipAddressStr + ipAddressStrLength;
uint8_t ipAddressArray[4];
for (int i = 0; i < 4; ++i) {
if (p == q) {
return false;
}
uint32_t part = 0;
for (int j = 0; j < 3; ++j) {
if (p == q) {
if (j > 0 && i == 3) {
break;
} else {
return false;
}
} else if (isDigit(*p)) {
part = part * 10 + (*p++ - '0');
} else if (j > 0 && *p == '.') {
break;
} else {
return false;
}
}
if (part > 255) {
return false;
}
if ((i < 3 && *p++ != '.') || (i == 3 && p != q)) {
return false;
}
ipAddressArray[i] = part;
}
ipAddress = arrayToIpAddress(ipAddressArray);
return true;
}
int getIpAddressPartA(uint32_t ipAddress) {
return ((uint8_t *)&ipAddress)[0];
}
void setIpAddressPartA(uint32_t *ipAddress, uint8_t value) {
((uint8_t *)ipAddress)[0] = value;
}
int getIpAddressPartB(uint32_t ipAddress) {
return ((uint8_t *)&ipAddress)[1];
}
void setIpAddressPartB(uint32_t *ipAddress, uint8_t value) {
((uint8_t *)ipAddress)[1] = value;
}
int getIpAddressPartC(uint32_t ipAddress) {
return ((uint8_t *)&ipAddress)[2];
}
void setIpAddressPartC(uint32_t *ipAddress, uint8_t value) {
((uint8_t *)ipAddress)[2] = value;
}
int getIpAddressPartD(uint32_t ipAddress) {
return ((uint8_t *)&ipAddress)[3];
}
void setIpAddressPartD(uint32_t *ipAddress, uint8_t value) {
((uint8_t *)ipAddress)[3] = value;
}
void ipAddressToArray(uint32_t ipAddress, uint8_t *ipAddressArray) {
ipAddressArray[0] = getIpAddressPartA(ipAddress);
ipAddressArray[1] = getIpAddressPartB(ipAddress);
ipAddressArray[2] = getIpAddressPartC(ipAddress);
ipAddressArray[3] = getIpAddressPartD(ipAddress);
}
uint32_t arrayToIpAddress(uint8_t *ipAddressArray) {
return getIpAddress(ipAddressArray[0], ipAddressArray[1], ipAddressArray[2], ipAddressArray[3]);
}
uint32_t getIpAddress(uint8_t a, uint8_t b, uint8_t c, uint8_t d) {
uint32_t ipAddress;
setIpAddressPartA(&ipAddress, a);
setIpAddressPartB(&ipAddress, b);
setIpAddressPartC(&ipAddress, c);
setIpAddressPartD(&ipAddress, d);
return ipAddress;
}
void ipAddressToString(uint32_t ipAddress, char *ipAddressStr, size_t maxIpAddressStrLength) {
snprintf(ipAddressStr, maxIpAddressStrLength, "%d.%d.%d.%d",
getIpAddressPartA(ipAddress), getIpAddressPartB(ipAddress),
getIpAddressPartC(ipAddress), getIpAddressPartD(ipAddress));
}
void macAddressToString(const uint8_t *macAddress, char *macAddressStr) {
for (int i = 0; i < 6; ++i) {
macAddressStr[3 * i] = toHexDigit((macAddress[i] & 0xF0) >> 4);
macAddressStr[3 * i + 1] = toHexDigit(macAddress[i] & 0xF);
macAddressStr[3 * i + 2] = i < 5 ? '-' : 0;
}
}
void formatTimeZone(int16_t timeZone, char *text, int count) {
if (timeZone == 0) {
stringCopy(text, count, "GMT");
} else {
char sign;
int16_t value;
if (timeZone > 0) {
sign = '+';
value = timeZone;
} else {
sign = '-';
value = -timeZone;
}
snprintf(text, count, "%c%02d:%02d GMT", sign, value / 100, value % 100);
}
}
bool parseTimeZone(const char *timeZoneStr, size_t timeZoneLength, int16_t &timeZone) {
int state = 0;
int sign = 1;
int integerPart = 0;
int fractionPart = 0;
const char *end = timeZoneStr + timeZoneLength;
for (const char *p = timeZoneStr; p < end; ++p) {
if (*p == ' ') {
continue;
}
if (state == 0) {
if (*p == '+') {
state = 1;
} else if (*p == '-') {
sign = -1;
state = 1;
} else if (isDigit(*p)) {
integerPart = *p - '0';
state = 2;
} else {
return false;
}
} else if (state == 1) {
if (isDigit(*p)) {
integerPart = (*p - '0');
state = 2;
} else {
return false;
}
} else if (state == 2) {
if (*p == ':') {
state = 4;
} else if (isDigit(*p)) {
integerPart = integerPart * 10 + (*p - '0');
state = 3;
} else {
return false;
}
} else if (state == 3) {
if (*p == ':') {
state = 4;
} else {
return false;
}
} else if (state == 4) {
if (isDigit(*p)) {
fractionPart = (*p - '0');
state = 5;
} else {
return false;
}
} else if (state == 5) {
if (isDigit(*p)) {
fractionPart = fractionPart * 10 + (*p - '0');
state = 6;
} else {
return false;
}
} else {
return false;
}
}
if (state != 2 && state != 3 && state != 6) {
return false;
}
int value = sign * (integerPart * 100 + fractionPart);
if (value < -1200 || value > 1400) {
return false;
}
timeZone = (int16_t)value;
return true;
}
void replaceCharacter(char *str, char ch, char repl) {
while (*str) {
if (*str == ch) {
*str = repl;
}
++str;
}
}
int strcicmp(char const *a, char const *b) {
for (;; a++, b++) {
int d = tolower((unsigned char)*a) - tolower((unsigned char)*b);
if (d != 0 || !*a)
return d;
}
}
int strncicmp(char const *a, char const *b, int n) {
for (; n--; a++, b++) {
int d = tolower((unsigned char)*a) - tolower((unsigned char)*b);
if (d != 0 || !*a)
return d;
}
return 0;
}
bool isStringEmpty(char const *s) {
for (; *s; s++) {
if (!isspace(*s)) {
return false;
}
}
return true;
}
bool startsWith(const char *str, const char *prefix) {
if (!str || !prefix)
return false;
size_t strLen = strlen(str);
size_t prefixLen = strlen(prefix);
if (prefixLen > strLen)
return false;
return strncmp(str, prefix, prefixLen) == 0;
}
bool startsWithNoCase(const char *str, const char *prefix) {
if (!str || !prefix)
return false;
size_t strLen = strlen(str);
size_t prefixLen = strlen(prefix);
if (prefixLen > strLen)
return false;
return strncicmp(str, prefix, prefixLen) == 0;
}
bool endsWith(const char *str, const char *suffix) {
if (!str || !suffix)
return false;
size_t strLen = strlen(str);
size_t suffixLen = strlen(suffix);
if (suffixLen > strLen)
return false;
return strncmp(str + strLen - suffixLen, suffix, suffixLen) == 0;
}
bool endsWithNoCase(const char *str, const char *suffix) {
if (!str || !suffix)
return false;
size_t strLen = strlen(str);
size_t suffixLen = strlen(suffix);
if (suffixLen > strLen)
return false;
return strncicmp(str + strLen - suffixLen, suffix, suffixLen) == 0;
}
void formatBytes(uint64_t bytes, char *text, int count) {
if (bytes == 0) {
stringCopy(text, count, "0 Bytes");
} else {
double c = 1024.0;
const char *e[] = { "Bytes", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB" };
uint64_t f = (uint64_t)floor(log((double)bytes) / log(c));
double g = round((bytes / pow(c, (double)f)) * 100) / 100;
snprintf(text, count, "%g %s", g, e[f]);
}
}
void getFileName(const char *path, char *fileName, unsigned fileNameSize) {
const char *a = strrchr(path, '/');
if (a) {
a++;
} else {
a = path;
}
const char *b = path + strlen(path);
unsigned n = b - a;
n = MIN(fileNameSize - 1, n);
if (n > 0) {
memcpy(fileName, a, n);
}
fileName[n] = 0;
}
void getBaseFileName(const char *path, char *baseName, unsigned baseNameSize) {
const char *a = strrchr(path, '/');
if (a) {
a++;
} else {
a = path;
}
const char *b = strrchr(path, '.');
if (!b || !(b >= a)) {
b = path + strlen(path);
}
unsigned n = b - a;
n = MIN(baseNameSize - 1, n);
if (n > 0) {
memcpy(baseName, a, n);
}
baseName[n] = 0;
}
////////////////////////////////////////////////////////////////////////////////
static const float PI = (float)M_PI;
static const float c1 = 1.70158f;
static const float c2 = c1 * 1.525f;
static const float c3 = c1 + 1.0f;
static const float c4 = (2 * PI) / 3;
static const float c5 = (2 * PI) / 4.5f;
float linear(float x) {
return x;
}
float easeInQuad(float x) {
return x * x;
}
float easeOutQuad(float x) {
return 1 - (1 - x) * (1 - x);
}
float easeInOutQuad(float x) {
return x < 0.5f ? 2 * x * x : 1 - powf(-2 * x + 2, 2) / 2;
}
float easeInCubic(float x) {
return x * x * x;
}
float easeOutCubic(float x) {
return 1 - pow(1 - x, 3);
}
float easeInOutCubic(float x) {
return x < 0.5f ? 4 * x * x * x : 1 - powf(-2 * x + 2, 3) / 2;
}
float easeInQuart(float x) {
return x * x * x * x;
}
float easeOutQuart(float x) {
return 1 - powf(1 - x, 4);
}
float easeInOutQuart(float x) {
return x < 0.5 ? 8 * x * x * x * x : 1 - powf(-2 * x + 2, 4) / 2;
}
float easeInQuint(float x) {
return x * x * x * x * x;
}
float easeOutQuint(float x) {
return 1 - powf(1 - x, 5);
}
float easeInOutQuint(float x) {
return x < 0.5f ? 16 * x * x * x * x * x : 1 - powf(-2 * x + 2, 5) / 2;
}
float easeInSine(float x) {
return 1 - cosf((x * PI) / 2);
}
float easeOutSine(float x) {
return sinf((x * PI) / 2);
}
float easeInOutSine(float x) {
return -(cosf(PI * x) - 1) / 2;
}
float easeInExpo(float x) {
return x == 0 ? 0 : powf(2, 10 * x - 10);
}
float easeOutExpo(float x) {
return x == 1 ? 1 : 1 - powf(2, -10 * x);
}
float easeInOutExpo(float x) {
return x == 0
? 0
: x == 1
? 1
: x < 0.5
? powf(2, 20 * x - 10) / 2
: (2 - powf(2, -20 * x + 10)) / 2;
}
float easeInCirc(float x) {
return 1 - sqrtf(1 - powf(x, 2));
}
float easeOutCirc(float x) {
return sqrtf(1 - powf(x - 1, 2));
}
float easeInOutCirc(float x) {
return x < 0.5
? (1 - sqrtf(1 - pow(2 * x, 2))) / 2
: (sqrtf(1 - powf(-2 * x + 2, 2)) + 1) / 2;
}
float easeInBack(float x) {
return c3 * x * x * x - c1 * x * x;
}
float easeOutBack(float x) {
return 1 + c3 * powf(x - 1, 3) + c1 * powf(x - 1, 2);
}
float easeInOutBack(float x) {
return x < 0.5
? (powf(2 * x, 2) * ((c2 + 1) * 2 * x - c2)) / 2
: (powf(2 * x - 2, 2) * ((c2 + 1) * (x * 2 - 2) + c2) + 2) / 2;
}
float easeInElastic(float x) {
return x == 0
? 0
: x == 1
? 1
: -powf(2, 10 * x - 10) * sinf((x * 10 - 10.75f) * c4);
}
float easeOutElastic(float x) {
return x == 0
? 0
: x == 1
? 1
: powf(2, -10 * x) * sinf((x * 10 - 0.75f) * c4) + 1;
}
float easeInOutElastic(float x) {
return x == 0
? 0
: x == 1
? 1
: x < 0.5
? -(powf(2, 20 * x - 10) * sinf((20 * x - 11.125f) * c5)) / 2
: (powf(2, -20 * x + 10) * sinf((20 * x - 11.125f) * c5)) / 2 + 1;
}
float easeOutBounce(float x);
float easeInBounce(float x) {
return 1 -easeOutBounce(1 - x);
}
float easeOutBounce(float x) {
static const float n1 = 7.5625f;
static const float d1 = 2.75f;
if (x < 1 / d1) {
return n1 * x * x;
} else if (x < 2 / d1) {
x -= 1.5f / d1;
return n1 * x * x + 0.75f;
} else if (x < 2.5f / d1) {
x -= 2.25f / d1;
return n1 * x * x + 0.9375f;
} else {
x -= 2.625f / d1;
return n1 * x * x + 0.984375f;
}
};
float easeInOutBounce(float x) {
return x < 0.5
? (1 - easeOutBounce(1 - 2 * x)) / 2
: (1 + easeOutBounce(2 * x - 1)) / 2;
}
EasingFuncType g_easingFuncs[] = {
linear,
easeInQuad,
easeOutQuad,
easeInOutQuad,
easeInCubic,
easeOutCubic,
easeInOutCubic,
easeInQuart,
easeOutQuart,
easeInOutQuart,
easeInQuint,
easeOutQuint,
easeInOutQuint,
easeInSine,
easeOutSine,
easeInOutSine,
easeInExpo,
easeOutExpo,
easeInOutExpo,
easeInCirc,
easeOutCirc,
easeInOutCirc,
easeInBack,
easeOutBack,
easeInOutBack,
easeInElastic,
easeOutElastic,
easeInOutElastic,
easeInBounce,
easeOutBounce,
easeInOutBounce,
};
} // namespace eez
#ifdef EEZ_PLATFORM_SIMULATOR_WIN32
char *strnstr(const char *s1, const char *s2, size_t n) {
char c = *s2;
if (c == '\0')
return (char *)s1;
for (size_t len = strlen(s2); len <= n; n--, s1++) {
if (*s1 == c) {
for (size_t i = 1;; i++) {
if (i == len) {
return (char *)s1;
}
if (s1[i] != s2[i]) {
break;
}
}
}
}
return NULL;
}
#endif