Add non-standard library calls used by String

Borrowed from https://github.com/esp8266/Arduino , include dtostrf() and
others.

Fixes #13

cores/rp2040/stdlib_noniso.cpp

+/*
+  stdlib_noniso.h - nonstandard (but useful) conversion functions
+
+  Copyright (c) 2014 Ivan Grokhotkov. All rights reserved.
+  Copyright (c) 2021 David Gauchard. All rights reserved.
+  This file is part of the esp8266 core for Arduino environment.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library 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
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#include <stdlib.h>
+#include <string.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <math.h>
+#include <limits>
+#include "stdlib_noniso.h"
+
+// ulltoa() is slower than std::to_char() (1.6 times)
+// but is smaller by ~800B/flash and ~250B/rodata
+
+// ulltoa fills str backwards and can return a pointer different from str
+extern "C" char* ulltoa(unsigned long long val, char* str, int slen, unsigned int radix)
+{
+    str += --slen;
+    *str = 0;
+    do
+    {
+        auto mod = val % radix;
+        val /= radix;
+        *--str = mod + ((mod > 9) ? ('a' - 10) : '0');
+    } while (--slen && val);
+    return val? nullptr: str;
+}
+
+// lltoa fills str backwards and can return a pointer different from str
+extern "C" char* lltoa (long long val, char* str, int slen, unsigned int radix)
+{
+    bool neg;
+    if (val < 0)
+    {
+        val = -val;
+        neg = true;
+    }
+    else
+    {
+        neg = false;
+    }
+    char* ret = ulltoa(val, str, slen, radix);
+    if (neg)
+    {
+        if (ret == str || ret == nullptr)
+            return nullptr;
+        *--ret = '-';
+    }
+    return ret;
+}
+
+extern "C" char* ltoa(long value, char* result, int base) {
+    return itoa((int)value, result, base);
+}
+
+extern "C" char* ultoa(unsigned long value, char* result, int base) {
+    return utoa((unsigned int)value, result, base);
+}
+
+extern "C" char * dtostrf(double number, signed char width, unsigned char prec, char *s) {
+    bool negative = false;
+
+    if (isnan(number)) {
+        strcpy(s, "nan");
+        return s;
+    }
+    if (isinf(number)) {
+        strcpy(s, "inf");
+        return s;
+    }
+
+    char* out = s;
+
+    int fillme = width; // how many cells to fill for the integer part
+    if (prec > 0) {
+        fillme -= (prec+1);
+    }
+
+    // Handle negative numbers
+    if (number < 0.0) {
+        negative = true;
+        fillme--;
+        number = -number;
+    }
+
+    // Round correctly so that print(1.999, 2) prints as "2.00"
+    // I optimized out most of the divisions
+    double rounding = 2.0;
+    for (uint8_t i = 0; i < prec; ++i)
+        rounding *= 10.0;
+    rounding = 1.0 / rounding;
+
+    number += rounding;
+
+    // Figure out how big our number really is
+    double tenpow = 1.0;
+    int digitcount = 1;
+    double nextpow;
+    while (number >= (nextpow = (10.0 * tenpow))) {
+        tenpow = nextpow;
+        digitcount++;
+    }
+
+    // minimal compensation for possible lack of precision (#7087 addition)
+    number *= 1 + std::numeric_limits<decltype(number)>::epsilon();
+
+    number /= tenpow;
+    fillme -= digitcount;
+
+    // Pad unused cells with spaces
+    while (fillme-- > 0) {
+        *out++ = ' ';
+    }
+
+    // Handle negative sign
+    if (negative) *out++ = '-';
+
+    // Print the digits, and if necessary, the decimal point
+    digitcount += prec;
+    int8_t digit = 0;
+    while (digitcount-- > 0) {
+        digit = (int8_t)number;
+        if (digit > 9) digit = 9; // insurance
+        *out++ = (char)('0' | digit);
+        if ((digitcount == prec) && (prec > 0)) {
+            *out++ = '.';
+        }
+        number -= digit;
+        number *= 10.0;
+    }
+
+    // make sure the string is terminated
+    *out = 0;
+    return s;
+}
+
+/*
+    strrstr (static)
+
+    Backwards search for p_pcPattern in p_pcString
+    Based on: https://stackoverflow.com/a/1634398/2778898
+
+*/
+extern "C" const char* strrstr(const char*__restrict p_pcString,
+                    const char*__restrict p_pcPattern)
+{
+    const char* pcResult = 0;
+
+    size_t      stStringLength = (p_pcString ? strlen(p_pcString) : 0);
+    size_t      stPatternLength = (p_pcPattern ? strlen(p_pcPattern) : 0);
+
+    if ((stStringLength) &&
+            (stPatternLength) &&
+            (stPatternLength <= stStringLength))
+    {
+        // Pattern is shorter or has the same length than the string
+        for (const char* s = (p_pcString + stStringLength - stPatternLength); s >= p_pcString; --s)
+        {
+            if (0 == strncmp(s, p_pcPattern, stPatternLength))
+            {
+                pcResult = s;
+                break;
+            }
+        }
+    }
+    return pcResult;
+}

cores/rp2040/stdlib_noniso.h

+/*
+  stdlib_noniso.h - nonstandard (but useful) conversion functions
+
+  Copyright (c) 2014 Ivan Grokhotkov. All rights reserved.
+  This file is part of the esp8266 core for Arduino environment.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library 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
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+#ifndef STDLIB_NONISO_H
+#define STDLIB_NONISO_H
+
+#ifdef __cplusplus
+extern "C"{
+#endif
+
+int atoi(const char *s);
+
+long atol(const char* s);
+
+double atof(const char* s);
+
+char* itoa (int val, char *s, int radix);
+
+char* ltoa (long val, char *s, int radix);
+
+char* lltoa (long long val, char* str, int slen, unsigned int radix);
+
+char* utoa (unsigned int val, char *s, int radix);
+
+char* ultoa (unsigned long val, char *s, int radix);
+
+char* ulltoa (unsigned long long val, char* str, int slen, unsigned int radix);
+
+char* dtostrf (double val, signed char width, unsigned char prec, char *s);
+
+void reverse(char* begin, char* end);
+
+const char* strrstr(const char*__restrict p_pcString,
+                    const char*__restrict p_pcPattern);
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+
+#endif