ManagedString.cpp

00001 /*
00002 The MIT License (MIT)
00003 
00004 Copyright (c) 2016 British Broadcasting Corporation.
00005 This software is provided by Lancaster University by arrangement with the BBC.
00006 
00007 Permission is hereby granted, free of charge, to any person obtaining a
00008 copy of this software and associated documentation files (the "Software"),
00009 to deal in the Software without restriction, including without limitation
00010 the rights to use, copy, modify, merge, publish, distribute, sublicense,
00011 and/or sell copies of the Software, and to permit persons to whom the
00012 Software is furnished to do so, subject to the following conditions:
00013 
00014 The above copyright notice and this permission notice shall be included in
00015 all copies or substantial portions of the Software.
00016 
00017 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
00018 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
00019 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
00020 THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
00021 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
00022 FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
00023 DEALINGS IN THE SOFTWARE.
00024 */
00025 
00026 /**
00027   * Class definition for a ManagedString.
00028   *
00029   * Uses basic reference counting to implement a copy-assignable, immutable string.
00030   *
00031   * This maps closely to the constructs found in many high level application languages,
00032   * such as Touch Develop.
00033   *
00034   * Written from first principles here, for several reasons:
00035   * 1) std::shared_ptr is not yet availiable on the ARMCC compiler
00036   *
00037   * 2) to reduce memory footprint - we don't need many of the other features in the std library
00038   *
00039   * 3) it makes an interesting case study for anyone interested in seeing how it works!
00040   *
00041   * 4) we need explicit reference counting to inter-op with low-level application langauge runtimes.
00042   *
00043   * 5) the reference counting needs to also work for read-only, flash-resident strings
00044   */
00045 #include <string.h>
00046 #include <stdlib.h>
00047 
00048 #include "mbed.h"
00049 #include "MicroBitConfig.h"
00050 #include "ManagedString.h"
00051 #include "MicroBitCompat.h"
00052 
00053 static const char empty[] __attribute__ ((aligned (4))) = "\xff\xff\0\0\0";
00054 
00055 /**
00056   * Internal constructor helper.
00057   *
00058   * Configures this ManagedString to refer to the static EmptyString
00059   */
00060 void ManagedString::initEmpty()
00061 {
00062     ptr = (StringData*)(void*)empty;
00063 }
00064 
00065 /**
00066   * Internal constructor helper.
00067   *
00068   * Creates this ManagedString based on a given null terminated char array.
00069   */
00070 void ManagedString::initString(const char *str)
00071 {
00072     // Initialise this ManagedString as a new string, using the data provided.
00073     // We assume the string is sane, and null terminated.
00074     int len = strlen(str);
00075     ptr = (StringData *) malloc(4+len+1);
00076     ptr->init();
00077     ptr->len = len;
00078     memcpy(ptr->data, str, len+1);
00079 }
00080 
00081 /**
00082   * Constructor.
00083   * Create a managed string from a specially prepared string literal.
00084   *
00085   * @param ptr The literal - first two bytes should be 0xff, then the length in little endian, then the literal. The literal has to be 4-byte aligned.
00086   *
00087   * @code
00088   * static const char hello[] __attribute__ ((aligned (4))) = "\xff\xff\x05\x00" "Hello";
00089   * ManagedString s((StringData*)(void*)hello);
00090   * @endcode
00091   */
00092 ManagedString::ManagedString(StringData *p)
00093 {
00094     ptr = p;
00095     ptr->incr();
00096 }
00097 
00098 /**
00099   * Get current ptr, do not decr() it, and set the current instance to empty string.
00100   *
00101   * This is to be used by specialized runtimes which pass StringData around.
00102   */
00103 StringData* ManagedString::leakData()
00104 {
00105     StringData *res = ptr;
00106     initEmpty();
00107     return res;
00108 }
00109 
00110 /**
00111   * Constructor.
00112   *
00113   * Create a managed string from a given integer.
00114   *
00115   * @param value The integer from which to create the ManagedString.
00116   *
00117   * @code
00118   * ManagedString s(20);
00119   * @endcode
00120   */
00121 ManagedString::ManagedString(const int value)
00122 {
00123     char str[12];
00124 
00125     itoa(value, str);
00126     initString(str);
00127 }
00128 
00129 /**
00130   * Constructor.
00131   * Create a managed string from a given char.
00132   *
00133   * @param value The character from which to create the ManagedString.
00134   *
00135   * @code
00136   * ManagedString s('a');
00137   * @endcode
00138   */
00139 ManagedString::ManagedString(const char value)
00140 {
00141     char str[2] = {value, 0};
00142     initString(str);
00143 }
00144 
00145 
00146 /**
00147   * Constructor.
00148   *
00149   * Create a managed string from a pointer to an 8-bit character buffer.
00150   *
00151   * The buffer is copied to ensure safe memory management (the supplied
00152   * character buffer may be declared on the stack for instance).
00153   *
00154   * @param str The character array on which to base the new ManagedString.
00155   *
00156   * @code
00157   * ManagedString s("abcdefg");
00158   * @endcode
00159   */
00160 ManagedString::ManagedString(const char *str)
00161 {
00162     // Sanity check. Return EmptyString for anything distasteful
00163     if (str == NULL || *str == 0)
00164     {
00165         initEmpty();
00166         return;
00167     }
00168 
00169     initString(str);
00170 }
00171 
00172 /**
00173   * Private Constructor.
00174   *
00175   * Create a managed string based on a concat of two strings.
00176   * The buffer is copied to ensure sane memory management (the supplied
00177   * character buffer may be declared on the stack for instance).
00178   *
00179   * @param str1 The first string on which to base the new ManagedString.
00180   *
00181   * @param str2 The second string on which to base the new ManagedString.
00182   */
00183 ManagedString::ManagedString(const ManagedString &s1, const ManagedString &s2)
00184 {
00185     // Calculate length of new string.
00186     int len = s1.length() + s2.length();
00187 
00188     // Create a new buffer for holding the new string data.
00189     ptr = (StringData*) malloc(4+len+1);
00190     ptr->init();
00191     ptr->len = len;
00192 
00193     // Enter the data, and terminate the string.
00194     memcpy(ptr->data, s1.toCharArray(), s1.length());
00195     memcpy(ptr->data + s1.length(), s2.toCharArray(), s2.length());
00196     ptr->data[len] = 0;
00197 }
00198 
00199 
00200 /**
00201   * Constructor.
00202   * Create a ManagedString from a PacketBuffer. All bytes in the
00203   * PacketBuffer are added to the ManagedString.
00204   *
00205   * @param buffer The PacktBuffer from which to create the ManagedString.
00206   *
00207   * @code
00208   * ManagedString s = radio.datagram.recv();
00209   * @endcode
00210   */
00211 ManagedString::ManagedString(PacketBuffer buffer)
00212 {
00213     // Allocate a new buffer ( just in case the data is not NULL terminated).
00214     ptr = (StringData*) malloc(4+buffer.length()+1);
00215     ptr->init();
00216 
00217     // Store the length of the new string
00218     ptr->len = buffer.length();
00219     memcpy(ptr->data, buffer.getBytes(), buffer.length());
00220     ptr->data[buffer.length()] = 0;
00221 }
00222 
00223 /**
00224   * Constructor.
00225   * Create a ManagedString from a pointer to an 8-bit character buffer of a given length.
00226   *
00227   * The buffer is copied to ensure sane memory management (the supplied
00228   * character buffer may be declared on the stack for instance).
00229   *
00230   * @param str The character array on which to base the new ManagedString.
00231   *
00232   * @param length The length of the character array
00233   *
00234   * @code
00235   * ManagedString s("abcdefg",7);
00236   * @endcode
00237   */
00238 ManagedString::ManagedString(const char *str, const int16_t length)
00239 {
00240     // Sanity check. Return EmptyString for anything distasteful
00241     if (str == NULL || *str == 0 || (uint16_t)length > strlen(str)) // XXX length should be unsigned on the interface
00242     {
00243         initEmpty();
00244         return;
00245     }
00246 
00247 
00248     // Allocate a new buffer, and create a NULL terminated string.
00249     ptr = (StringData*) malloc(4+length+1);
00250     ptr->init();
00251     // Store the length of the new string
00252     ptr->len = length;
00253     memcpy(ptr->data, str, length);
00254     ptr->data[length] = 0;
00255 }
00256 
00257 /**
00258   * Copy constructor.
00259   * Makes a new ManagedString identical to the one supplied.
00260   *
00261   * Shares the character buffer and reference count with the supplied ManagedString.
00262   *
00263   * @param s The ManagedString to copy.
00264   *
00265   * @code
00266   * ManagedString s("abcdefg");
00267   * ManagedString p(s);
00268   * @endcode
00269   */
00270 ManagedString::ManagedString(const ManagedString &s)
00271 {
00272     ptr = s.ptr;
00273     ptr->incr();
00274 }
00275 
00276 
00277 /**
00278   * Default constructor.
00279   *
00280   * Create an empty ManagedString.
00281   *
00282   * @code
00283   * ManagedString s();
00284   * @endcode
00285   */
00286 ManagedString::ManagedString()
00287 {
00288     initEmpty();
00289 }
00290 
00291 /**
00292   * Destructor.
00293   *
00294   * Free this ManagedString, and decrement the reference count to the
00295   * internal character buffer.
00296   *
00297   * If we're holding the last reference, also free the character buffer.
00298   */
00299 ManagedString::~ManagedString()
00300 {
00301     ptr->decr();
00302 }
00303 
00304 /**
00305   * Copy assign operation.
00306   *
00307   * Called when one ManagedString is assigned the value of another.
00308   *
00309   * If the ManagedString being assigned is already referring to a character buffer,
00310   * decrement the reference count and free up the buffer as necessary.
00311   *
00312   * Then, update our character buffer to refer to that of the supplied ManagedString,
00313   * and increase its reference count.
00314   *
00315   * @param s The ManagedString to copy.
00316   *
00317   * @code
00318   * ManagedString s("abcd");
00319   * ManagedString p("efgh");
00320   * p = s   // p now points to s, s' ref is incremented
00321   * @endcode
00322   */
00323 ManagedString& ManagedString::operator = (const ManagedString& s)
00324 {
00325     if (this->ptr == s.ptr)
00326         return *this;
00327 
00328     ptr->decr();
00329     ptr = s.ptr;
00330     ptr->incr();
00331 
00332     return *this;
00333 }
00334 
00335 /**
00336   * Equality operation.
00337   *
00338   * Called when one ManagedString is tested to be equal to another using the '==' operator.
00339   *
00340   * @param s The ManagedString to test ourselves against.
00341   *
00342   * @return true if this ManagedString is identical to the one supplied, false otherwise.
00343   *
00344   * @code
00345   * MicroBitDisplay display;
00346   * ManagedString s("abcd");
00347   * ManagedString p("efgh");
00348   *
00349   * if(p == s)
00350   *     display.scroll("We are the same!");
00351   * else
00352   *     display.scroll("We are different!"); //p is not equal to s - this will be called
00353   * @endcode
00354   */
00355 bool ManagedString::operator== (const ManagedString& s)
00356 {
00357     return ((length() == s.length()) && (strcmp(toCharArray(),s.toCharArray())==0));
00358 }
00359 
00360 /**
00361   * Inequality operation.
00362   *
00363   * Called when one ManagedString is tested to be less than another using the '<' operator.
00364   *
00365   * @param s The ManagedString to test ourselves against.
00366   *
00367   * @return true if this ManagedString is alphabetically less than to the one supplied, false otherwise.
00368   *
00369   * @code
00370   * MicroBitDisplay display;
00371   * ManagedString s("a");
00372   * ManagedString p("b");
00373   *
00374   * if(s < p)
00375   *     display.scroll("a is before b!"); //a is before b
00376   * else
00377   *     display.scroll("b is before a!");
00378   * @endcode
00379   */
00380 bool ManagedString::operator< (const ManagedString& s)
00381 {
00382     return (strcmp(toCharArray(), s.toCharArray())<0);
00383 }
00384 
00385 /**
00386   * Inequality operation.
00387   *
00388   * Called when one ManagedString is tested to be greater than another using the '>' operator.
00389   *
00390   * @param s The ManagedString to test ourselves against.
00391   *
00392   * @return true if this ManagedString is alphabetically greater than to the one supplied, false otherwise.
00393   *
00394   * @code
00395   * MicroBitDisplay display;
00396   * ManagedString s("a");
00397   * ManagedString p("b");
00398   *
00399   * if(p>a)
00400   *     display.scroll("b is after a!"); //b is after a
00401   * else
00402   *     display.scroll("a is after b!");
00403   * @endcode
00404   */
00405 bool ManagedString::operator> (const ManagedString& s)
00406 {
00407     return (strcmp(toCharArray(), s.toCharArray())>0);
00408 }
00409 
00410 /**
00411   * Extracts a ManagedString from this string, at the position provided.
00412   *
00413   * @param start The index of the first character to extract, indexed from zero.
00414   *
00415   * @param length The number of characters to extract from the start position
00416   *
00417   * @return a ManagedString representing the requested substring.
00418   *
00419   * @code
00420   * MicroBitDisplay display;
00421   * ManagedString s("abcdefg");
00422   *
00423   * display.scroll(s.substring(0,2)) // displays "ab"
00424   * @endcode
00425   */
00426 ManagedString ManagedString::substring(int16_t start, int16_t length)
00427 {
00428     // If the parameters are illegal, just return a reference to the empty string.
00429     if (start >= this->length())
00430         return ManagedString(ManagedString::EmptyString);
00431 
00432     // Compute a safe copy length;
00433     length = min(this->length()-start, length);
00434 
00435     // Build a ManagedString from this.
00436     return ManagedString(toCharArray()+start, length);
00437 }
00438 
00439 /**
00440   * Concatenates two strings.
00441   *
00442   * @param lhs The first ManagedString to concatenate.
00443   * @param rhs The second ManagedString to concatenate.
00444   *
00445   * @return a new ManagedString representing the joined strings.
00446   *
00447   * @code
00448   * MicroBitDisplay display;
00449   * ManagedString s("abcd");
00450   * ManagedString p("efgh")
00451   *
00452   * display.scroll(s + p) // scrolls "abcdefgh"
00453   * @endcode
00454   */
00455 ManagedString operator+ (const ManagedString& lhs, const ManagedString& rhs)
00456 {
00457 
00458     // If the either string is empty, nothing to do!
00459     if (rhs.length() == 0)
00460         return lhs;
00461 
00462     if (lhs.length() == 0)
00463         return rhs;
00464 
00465     return ManagedString(lhs, rhs);
00466 }
00467 
00468 
00469 /**
00470   * Provides a character value at a given position in the string, indexed from zero.
00471   *
00472   * @param index The position of the character to return.
00473   *
00474   * @return the character at position index, zero if index is invalid.
00475   *
00476   * @code
00477   * MicroBitDisplay display;
00478   * ManagedString s("abcd");
00479   *
00480   * display.scroll(s.charAt(1)) // scrolls "b"
00481   * @endcode
00482   */
00483 char ManagedString::charAt(int16_t index)
00484 {
00485     return (index >=0 && index < length()) ? ptr->data[index] : 0;
00486 }
00487 
00488 /**
00489   * Empty string constant literal
00490   */
00491 ManagedString ManagedString::EmptyString((StringData*)(void*)empty);