CS107 Handout 26 Spring 2007 May 9, 2007 CS107 Midterm Exam

Transcription

1 CS107 Handout 26 Spring 2007 May 9, 2007 CS107 Midterm Exam This is an open-note exam. You can refer to any course handouts, handwritten lecture notes, and printouts of any code relevant to a CS107 assignment. You may not use any laptops, cell phones, or handheld devices of any sort. Those taking the exam remotely should phone in if they have questions. Once you re done, fax the exam to Stanford, but hold on to the original until you get the graded fax copy back. Cell phone is , and fax number is Good luck! leland username: Last Name: First Name: I accept the letter and spirit of the honor code. I ve neither given nor received aid on this exam. I promise to write as neatly as possible. (signed) Score Grader 1. Color Theory (15) 2. Packing Packets (15) 3. Python (15) Total (15)

2 vector.h typedef int (*VectorCompareFunction)(const void *elemaddr1, const void *elemaddr2); typedef void (*VectorMapFunction)(void *elemaddr, void *auxdata); typedef void (*VectorFreeFunction)(void *elemaddr); typedef struct { // implementation specific } vector; void VectorNew(vector *v, int elemsize, VectorFreeFunction freefn, int initialallocation); void VectorDispose(vector *v); int VectorLength(vector *v); void *VectorNth(vector *v, int position); void VectorInsert(vector *v, const void *elemaddr, int position); void VectorAppend(vector *v, const void *elemaddr); void VectorReplace(vector *v, const void *elemaddr, int position); void VectorDelete(vector *v, int position); int VectorSearch(vector *v, const void *key, VectorCompareFunction searchfn, int startindex, bool issorted); void VectorSort(vector *v, VectorCompareFunction comparefn); void VectorMap(vector *v, VectorMapFunction mapfn, void *auxdata); hashset.h typedef int (*HashSetHashFunction)(const void *elemaddr, int numbuckets); typedef int (*HashSetCompareFunction)(const void *elemaddr1, const void *elemaddr2); typedef void (*HashSetMapFunction)(void *elemaddr, void *auxdata); typedef void (*HashSetFreeFunction)(void *elemaddr); typedef struct { // implementation specific } hashset; void HashSetNew(hashset *h, int elemsize, int numbuckets, HashSetHashFunction hashfn, HashSetCompareFunction comparefn, HashSetFreeFunction freefn); void HashSetDispose(hashset *h); int HashSetCount(hashset *h); void HashSetEnter(hashset *h, const void *elemaddr); void *HashSetLookup(hashset *h, const void *elemaddr); void HashSetMap(hashset *h, HashSetMapFunction mapfn, void *auxdata); 2 tear me out

3 3 Problem 1: Color Theory (15 points; 5, 5, and 5) Consider the following struct definitions: typedef struct palette { short *lavender; char indigo[4]; struct palette *peach[2]; } palette; line 1 line 2 line 3 static palette **swatch(palette *ecru, int mint); static int colorwheel(palette ruby, int seafoam) { seafoam = ruby.lavender[ruby.indigo[3]]; ruby.peach[0] += ((palette *)ruby.indigo)->lavender[2]; return swatch(&ruby, 8) - ruby.peach; } Generate code for the entire colorwheel function. Be clear about what assembly code corresponds to what line. You have this and the next two pages for your work.

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6 6 Problem 2: packpackets (15 points) You re to write code that traverses a linked list of custom nodes and builds a dynamically allocated array of bytes large enough to store the meaningful data held by the linked list. Each node stores one or more packets of raw information, where each packet is preceded by a two-byte short stating the length (in bytes) of the packet that follows it. The end of the packet sequence is marked by a two-byte zero, and the four bytes after that store the address of the next node in the list (or NULL, if the node is the last in the list). The following node contains three packets of length 20, 10, and 30 bytes, in that order. The number of bytes making up the entire node is You re to write a function called packpackets that, given the address of the first node in such a list, builds a dynamically allocated byte array where all packets in the linked list are replicated verbatim, one after another, in the same order they appear in the list. So, given the following list: NULL your packpackets function would return the address of the first byte of this dynamically allocated figure: Here are some constraints I m imposing on your implementation. The address passed to packpackets is the address of the first short storing the number of bytes making up the first packet in the first node. Each node alternates between shorts and packets. A two-byte zero marks the end of the packet sequence in any given node. Every node ends with a four-byte pointer identifying the location of the next node in the list. You should implement this function iteratively, and your algorithm should accomplish everything in exactly one traversal. This requires you to call realloc for each packet you encounter during your one traversal. [Tip: When NULL is passed as the first argument to realloc, realloc just calls malloc on your behalf.] This artificial constraint is being imposed so that I can test your understanding of realloc. For simplicity, you may assume the list contains at least one node, and that each node contains at least one packet. You needn t worry about any alignment restrictions at all. You shouldn t free or modify the original list in any way. Use the next page for your implementation. Feel free to tear this page out.

7 7 Problem 2 Continued /** * Function: packpackets * * Builds a contiguous array version of the packet list structure according * to the explanations provided on the prior page. The parameter passed * is of type short *, because the first meaningful piece of information * stored in the list is a two-byte short. The return value is of type * void *, because the implementation has no type information about the * packet data. */ void *packpackets(short *list) {

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9 9 Problem 3: Python (15 points) Python provides support for objects just like C++ and Java do, but its implementation model is different. Rather than requiring that all fields be stated at compile time, Python allows its objects to take on new fields as the code executes. Here s how a Python object might grow to represent a Stanford student: student = {} # initialized to be an empty object student['name'] = 'Edward Requenez' student['numunits'] = 15 student['gpa'] = student['friends'] = ['Bill','Will','Sawyer','Alex','Daniel'] student['hometown'] = {} # initialized to be an empty object student['hometown']['city'] = 'Dallas' student['hometown']['state'] = 'Texas' student['hometown']['zip'] = You don t need to know Python to know what the code s trying to do. The coder decided to model some Stanford student as a Python object with five properties. Each accommodates a value of a different data type. Python doesn t require that object structure be stated up front. We re going to assume that all Python values whether they re top-level primitives or fields within an aggregate record, are implemented in terms of the following pure C enum and struct definitions: typedef enum { Integer, Float, String, Array, Object } PythonType; typedef struct { const char *id; PythonType type; char value[4]; } PythonPair; Don t be misled PythonPair's third field. It looks like it stores a small number of characters, but it actually sits there as a general-purpose four-byte register of sorts. We ve done enough C casting and reinterpretation of bit patterns to know how to exploit that little value array for all it s worth. Sometimes we used those four bytes to store an integer, as with: numdalmations = 101 For other primitives, we use the four bytes to store something else: pi = ; contestant = 'Sanjaya Malakar' The value array contains the bit pattern of either a four-byte integer, a four-byte float or a four-byte pointer to a null-terminated char array. We rely on the type field to figure out what those four bytes are storing Sanjaya Malakar\0 Integer Float String numdalmations\0 pi\0 contestant\0

10 And guess what? Python objects and arrays are backed by the infamous hashset and vector types from Assignments 3 and When a field is an array, the value array stores the address of a vector of PythonPairs, where the id fields just store NULL as a sentinel that neither the id fields nor the colons need to be printed. When a field is an inner object, value stores the address of a hashset. The hashset is the in-memory representation of a Python object that s been built up to include an arbitrarily large number of properties. Those properties are stored as PythonPairs in a hashset, and provided you allow for some of those PythonPairss to store vector *s and hashset *s, you have the means to recursively build Python objects with primitives, Python arrays, and Python objects as fields. Given the address of a hashset of PythonPairss, you re going to write a function called serializeobject that synthesizes the C string representation of the entire object, just like Python would. Primitives are serialized as you d expect, though all strings constants are delimited by single quotes. Arrays are square-bracket-delimited, comma-separated lists. The student object from above looks like this: {'name':'ed Requenez','numUnits':15,'gpa':3.966, 'friends':['bill','will','sawyer','alex','daniel',], 'hometown':{'city':'dallas','state':'texas','zipcode':75209,},} A full object s serialization is delimited by curly braces. Between those curly braces is a comma-separated list of id/value pairs. The identifiers are always strings with singe quotes. Each value gets published either as the primitive it is, or recursively as the array or sub-object it s addressing. The identifiers are separated from their values by colons, there s no intervening white space, and it s fine to have a trailing comma after the last entry within an array of object serialization. This is a difficult question that necessarily uses HashSetMap. And because HashSetMapFunction and VectorMapFunction have the same prototype, it makes sense use VectorMap to serialize Python arrays. You may assume the existence of convenience functions called intcpy and floatcpy, which operate much like strcpy, in that they place '\0'-terminated C strings in the specified buffer. In this case the strings they place are the textual equivalents of integers and floats. void intcpy(char *dest, int value); // does not allocate memory void floatcpy(char *dest, float value); // does not allocate memory Use the space provided on the next three pages for your implementation. You re more than encouraged to decompose your solution. Don t orphan any memory. For simplicity, you may assume that the full serialization is less than 8192 characters (a nice power of 2), but the dynamically allocated string that s ultimately returned must use only as much memory as is absolutely necessary to store the serialization.

11 11 Problem 3 Continued /** * Function: serializeobject * * Traverses the hashset representation of a Python object and * synthesizes a single, dynamically allocated C string storing the * textual representation of the entire object. */ char *serializeobject(hashset *object) {

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