ManagedString.h

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 #ifndef MANAGED_STRING_H
00027 #define MANAGED_STRING_H
00028 
00029 #include "MicroBitConfig.h"
00030 #include "RefCounted.h"
00031 #include "PacketBuffer.h"
00032 
00033 struct StringData : RefCounted
00034 {
00035     uint16_t len;
00036     char data[0];
00037 };
00038 
00039 
00040 /**
00041   * Class definition for a ManagedString.
00042   *
00043   * Uses basic reference counting to implement a copy-assignable, immutable string.
00044   *
00045   * This maps closely to the constructs found in many high level application languages,
00046   * such as Touch Develop.
00047   *
00048   * Written from first principles here, for several reasons:
00049   * 1) std::shared_ptr is not yet availiable on the ARMCC compiler
00050   *
00051   * 2) to reduce memory footprint - we don't need many of the other features in the std library
00052   *
00053   * 3) it makes an interesting case study for anyone interested in seeing how it works!
00054   *
00055   * 4) we need explicit reference counting to inter-op with low-level application langauge runtimes.
00056   *
00057   * 5) the reference counting needs to also work for read-only, flash-resident strings
00058   */
00059 class ManagedString
00060 {
00061     // StringData contains the reference count, the length, follwed by char[] data, all in one block.
00062     // When referece count is 0xffff, then it's read only and should not be counted.
00063     // Otherwise the block was malloc()ed.
00064     // We control access to this to proide immutability and reference counting.
00065     StringData *ptr;
00066 
00067     public:
00068 
00069     /**
00070       * Constructor.
00071       * Create a managed string from a specially prepared string literal.
00072       *
00073       * @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.
00074       *
00075       * @code
00076       * static const char hello[] __attribute__ ((aligned (4))) = "\xff\xff\x05\x00" "Hello";
00077       * ManagedString s((StringData*)(void*)hello);
00078       * @endcode
00079       */
00080     ManagedString(StringData *ptr);
00081 
00082     /**
00083       * Get current ptr, do not decr() it, and set the current instance to empty string.
00084       *
00085       * This is to be used by specialized runtimes which pass StringData around.
00086       */
00087     StringData *leakData();
00088 
00089     /**
00090       * Constructor.
00091       *
00092       * Create a managed string from a pointer to an 8-bit character buffer.
00093       *
00094       * The buffer is copied to ensure safe memory management (the supplied
00095       * character buffer may be declared on the stack for instance).
00096       *
00097       * @param str The character array on which to base the new ManagedString.
00098       *
00099       * @code
00100       * ManagedString s("abcdefg");
00101       * @endcode
00102       */
00103     ManagedString(const char *str);
00104 
00105     /**
00106       * Constructor.
00107       *
00108       * Create a managed string from a given integer.
00109       *
00110       * @param value The integer from which to create the ManagedString.
00111       *
00112       * @code
00113       * ManagedString s(20);
00114       * @endcode
00115       */
00116     ManagedString(const int value);
00117 
00118 
00119     /**
00120       * Constructor.
00121       * Create a managed string from a given char.
00122       *
00123       * @param value The character from which to create the ManagedString.
00124       *
00125       * @code
00126       * ManagedString s('a');
00127       * @endcode
00128       */
00129     ManagedString(const char value);
00130 
00131     /**
00132       * Constructor.
00133       * Create a ManagedString from a PacketBuffer. All bytes in the
00134       * PacketBuffer are added to the ManagedString.
00135       *
00136       * @param buffer The PacktBuffer from which to create the ManagedString.
00137       *
00138       * @code
00139       * ManagedString s = radio.datagram.recv();
00140       * @endcode
00141       */
00142     ManagedString(PacketBuffer buffer);
00143 
00144     /**
00145       * Constructor.
00146       * Create a ManagedString from a pointer to an 8-bit character buffer of a given length.
00147       *
00148       * The buffer is copied to ensure sane memory management (the supplied
00149       * character buffer may be declared on the stack for instance).
00150       *
00151       * @param str The character array on which to base the new ManagedString.
00152       *
00153       * @param length The length of the character array
00154       *
00155       * @code
00156       * ManagedString s("abcdefg",7);
00157       * @endcode
00158       */
00159     ManagedString(const char *str, const int16_t length);
00160 
00161     /**
00162       * Copy constructor.
00163       * Makes a new ManagedString identical to the one supplied.
00164       *
00165       * Shares the character buffer and reference count with the supplied ManagedString.
00166       *
00167       * @param s The ManagedString to copy.
00168       *
00169       * @code
00170       * ManagedString s("abcdefg");
00171       * ManagedString p(s);
00172       * @endcode
00173       */
00174     ManagedString(const ManagedString &s);
00175 
00176     /**
00177       * Default constructor.
00178       *
00179       * Create an empty ManagedString.
00180       *
00181       * @code
00182       * ManagedString s();
00183       * @endcode
00184       */
00185     ManagedString();
00186 
00187     /**
00188       * Destructor.
00189       *
00190       * Free this ManagedString, and decrement the reference count to the
00191       * internal character buffer.
00192       *
00193       * If we're holding the last reference, also free the character buffer.
00194       */
00195     ~ManagedString();
00196 
00197     /**
00198       * Copy assign operation.
00199       *
00200       * Called when one ManagedString is assigned the value of another.
00201       *
00202       * If the ManagedString being assigned is already referring to a character buffer,
00203       * decrement the reference count and free up the buffer as necessary.
00204       *
00205       * Then, update our character buffer to refer to that of the supplied ManagedString,
00206       * and increase its reference count.
00207       *
00208       * @param s The ManagedString to copy.
00209       *
00210       * @code
00211       * ManagedString s("abcd");
00212       * ManagedString p("efgh");
00213       * p = s   // p now points to s, s' ref is incremented
00214       * @endcode
00215       */
00216     ManagedString& operator = (const ManagedString& s);
00217 
00218     /**
00219       * Equality operation.
00220       *
00221       * Called when one ManagedString is tested to be equal to another using the '==' operator.
00222       *
00223       * @param s The ManagedString to test ourselves against.
00224       *
00225       * @return true if this ManagedString is identical to the one supplied, false otherwise.
00226       *
00227       * @code
00228       * MicroBitDisplay display;
00229       * ManagedString s("abcd");
00230       * ManagedString p("efgh");
00231       *
00232       * if(p == s)
00233       *     display.scroll("We are the same!");
00234       * else
00235       *     display.scroll("We are different!"); //p is not equal to s - this will be called
00236       * @endcode
00237       */
00238     bool operator== (const ManagedString& s);
00239 
00240     /**
00241       * Inequality operation.
00242       *
00243       * Called when one ManagedString is tested to be less than another using the '<' operator.
00244       *
00245       * @param s The ManagedString to test ourselves against.
00246       *
00247       * @return true if this ManagedString is alphabetically less than to the one supplied, false otherwise.
00248       *
00249       * @code
00250       * MicroBitDisplay display;
00251       * ManagedString s("a");
00252       * ManagedString p("b");
00253       *
00254       * if(s < p)
00255       *     display.scroll("a is before b!"); //a is before b
00256       * else
00257       *     display.scroll("b is before a!");
00258       * @endcode
00259       */
00260     bool operator< (const ManagedString& s);
00261 
00262     /**
00263       * Inequality operation.
00264       *
00265       * Called when one ManagedString is tested to be greater than another using the '>' operator.
00266       *
00267       * @param s The ManagedString to test ourselves against.
00268       *
00269       * @return true if this ManagedString is alphabetically greater than to the one supplied, false otherwise.
00270       *
00271       * @code
00272       * MicroBitDisplay display;
00273       * ManagedString s("a");
00274       * ManagedString p("b");
00275       *
00276       * if(p>a)
00277       *     display.scroll("b is after a!"); //b is after a
00278       * else
00279       *     display.scroll("a is after b!");
00280       * @endcode
00281       */
00282     bool operator> (const ManagedString& s);
00283 
00284     /**
00285       * Extracts a ManagedString from this string, at the position provided.
00286       *
00287       * @param start The index of the first character to extract, indexed from zero.
00288       *
00289       * @param length The number of characters to extract from the start position
00290       *
00291       * @return a ManagedString representing the requested substring.
00292       *
00293       * @code
00294       * MicroBitDisplay display;
00295       * ManagedString s("abcdefg");
00296       *
00297       * display.scroll(s.substring(0,2)) // displays "ab"
00298       * @endcode
00299       */
00300     ManagedString substring(int16_t start, int16_t length);
00301 
00302     /**
00303       * Concatenates two strings.
00304       *
00305       * @param lhs The first ManagedString to concatenate.
00306       * @param rhs The second ManagedString to concatenate.
00307       *
00308       * @return a new ManagedString representing the joined strings.
00309       *
00310       * @code
00311       * MicroBitDisplay display;
00312       * ManagedString s("abcd");
00313       * ManagedString p("efgh")
00314       *
00315       * display.scroll(s + p) // scrolls "abcdefgh"
00316       * @endcode
00317       */
00318     friend ManagedString operator+ (const ManagedString& lhs, const ManagedString& rhs);
00319 
00320     /**
00321       * Provides a character value at a given position in the string, indexed from zero.
00322       *
00323       * @param index The position of the character to return.
00324       *
00325       * @return the character at posisiton index, zero if index is invalid.
00326       *
00327       * @code
00328       * MicroBitDisplay display;
00329       * ManagedString s("abcd");
00330       *
00331       * display.scroll(s.charAt(1)) // scrolls "b"
00332       * @endcode
00333       */
00334     char charAt(int16_t index);
00335 
00336 
00337     /**
00338       * Provides an immutable 8 bit wide character buffer representing this string.
00339       *
00340       * @return a pointer to the character buffer.
00341       */
00342     const char *toCharArray() const
00343     {
00344         return ptr->data;
00345     }
00346 
00347     /**
00348       * Determines the length of this ManagedString in characters.
00349       *
00350       * @return the length of the string in characters.
00351       *
00352       * @code
00353       * MicroBitDisplay display;
00354       * ManagedString s("abcd");
00355       *
00356       * display.scroll(s.length()) // scrolls "4"
00357       * @endcode
00358       */
00359     int16_t length() const
00360     {
00361         return ptr->len;
00362     }
00363 
00364     /**
00365       * Empty String constant
00366       */
00367     static ManagedString EmptyString;
00368 
00369     private:
00370 
00371     /**
00372       * Internal constructor helper.
00373       *
00374       * Configures this ManagedString to refer to the static EmptyString
00375       */
00376     void initEmpty();
00377 
00378     /**
00379       * Internal constructor helper.
00380       *
00381       * Creates this ManagedString based on a given null terminated char array.
00382       */
00383     void initString(const char *str);
00384 
00385     /**
00386       * Private Constructor.
00387       *
00388       * Create a managed string based on a concat of two strings.
00389       * The buffer is copied to ensure sane memory management (the supplied
00390       * character buffer may be declared on the stack for instance).
00391       *
00392       * @param str1 The first string on which to base the new ManagedString.
00393       *
00394       * @param str2 The second string on which to base the new ManagedString.
00395       */
00396     ManagedString(const ManagedString &s1, const ManagedString &s2);
00397 
00398 };
00399 
00400 #endif