hashcode.c

#include <string.h>
#include <stdint.h>
#include <stdlib.h>
#include <limits.h>
#include "hashcode.h"

int ipow(int base, int exp) {
  int result = 1;
  while (exp) {
    if (exp & 1)
      result *= base;
    exp >>= 1;
    base *= base;
  }

  return result;
}

int java_hash_code(const char *str) {
  int sum = 0;
  int len = strlen(str);
  int i = len;

  while(--i > -1) {
    sum += str[i] * ipow(31, (len - (i + 1)));
  }
  
  return sum;
}

// http://blogs.sun.com/darcy/entry/string_unhashing?intcmp=2223
char *java_unhash_code(int target) {
    // leaky leaky
    char *result = calloc(100, sizeof(char));

    if (target < 0) {
        // "polygenelubricants" is known to hash to INT_MIN
        strcat(result, "polygenelubricants");

        if (target == INT_MIN)
            return result;

        target &= INT_MAX;
    }

    unhash2(result, target);
    return result;
}

// assumes target is positive
void unhash2(char *partial, int target) {
  int div = target / 31;
  int rem = target % 31;
  int len = strlen(partial);

  if (div <= CHAR_MAX) {
      if (div != 0) {
          partial[len] = div;
          partial[++len] = '\0';
      }

      partial[len] = rem;
      partial[len + 1] = '\0';
  } else {
      unhash2(partial, div);
      len = strlen(partial);
      partial[len] = rem;
      partial[len + 1] = '\0';
  }
}

hashcode.h

#ifndef _HASHCODE_H_
#define _HASHCODE_H_

int ipow(int base, int exp);
int java_hash_code(const char *str);
char *java_unhash_code(int target);
void unhash2(char *partial, int target);

#endif

Makefile

SHELL=/bin/sh
CC=clang
SOURCES=hashcode.c
OBJECTS=hashcode.o
LIBFILE=libhashcode.so
LIBVERSION=0
SHARED=$(LIBFILE).0.0.1
LINKS=$(LIBFILE) $(LIBFILE).$(LIBVERSION)

CC_OBJECTS_OPTS=-c -fPIC -O3 -Wall -Werror -pedantic-errors -std=c99
CC_SHARED_OPTS=-shared -Wl,-soname,$(LIBFILE).$(LIBVERSION)

INSTALL_DIR=../../lib/ext

LINK_CMD=ln -fs

.PHONY: all install clean

all: $(SOURCES) $(OBJECTS) $(SHARED)

install: $(SHARED)
	mkdir -p $(INSTALL_DIR)
	install $(SHARED) $(INSTALL_DIR)
	$(LINK_CMD) $(INSTALL_DIR)/$(SHARED) $(INSTALL_DIR)/$(LIBFILE).$(LIBVERSION)
	$(LINK_CMD) $(INSTALL_DIR)/$(SHARED) $(INSTALL_DIR)/$(LIBFILE)

$(SHARED): $(OBJECTS)
	$(CC) $(CC_SHARED_OPTS) -o $@ $(OBJECTS) 

$(OBJECTS): $(SOURCES)
	$(CC) $(CC_OBJECTS_OPTS) $(SOURCES) -o $@

clean:
	rm -f *.o *.so* core *.core