// Copyright 2011-2022 David Robillard <d@drobilla.net>
// SPDX-License-Identifier: ISC

#include "symap.h"

#include <assert.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

/**
  @file symap.c Implementation of Symap, a basic symbol map (string interner).

  This implementation is primitive, but has some desirable qualities: good
  (O(lg(n)) lookup performance for already-mapped symbols, minimal space
  overhead, extremely fast (O(1)) reverse mapping (ID to string), simple code,
  no dependencies.

  The tradeoff is that mapping new symbols may be quite slow.  In other words,
  this implementation is ideal for use cases with a relatively limited set of
  symbols, or where most symbols are mapped early.  It will not fare so well
  with very dynamic sets of symbols.  For that, you're better off with a
  tree-based implementation (and the associated space cost, especially if you
  need reverse mapping).
*/

struct SymapImpl {
  /// Unsorted array of strings, so symbols[i] is the symbol for ID i
  char** symbols;

  /// Array of IDs, sorted by corresponding string in `symbols`
  uint32_t* index;

  /// Number of symbols (number of items in `symbols` and `index`)
  uint32_t size;
};

Symap*
symap_new(void)
{
  Symap* map   = (Symap*)malloc(sizeof(Symap));
  map->symbols = NULL;
  map->index   = NULL;
  map->size    = 0;
  return map;
}

void
symap_free(Symap* map)
{
  if (!map) {
    return;
  }

  for (uint32_t i = 0; i < map->size; ++i) {
    free(map->symbols[i]);
  }

  free(map->symbols);
  free(map->index);
  free(map);
}

static char*
symap_strdup(const char* str)
{
  const size_t len  = strlen(str);
  char*        copy = (char*)malloc(len + 1);
  memcpy(copy, str, len + 1);
  return copy;
}

/**
   Return the index into map->index (not the ID) corresponding to `sym`,
   or the index where a new entry for `sym` should be inserted.
*/
static uint32_t
symap_search(const Symap* map, const char* sym, bool* exact)
{
  *exact = false;
  if (map->size == 0) {
    return 0; // Empty map, insert at 0
  }

  if (strcmp(map->symbols[map->index[map->size - 1] - 1], sym) < 0) {
    return map->size; // Greater than last element, append
  }

  uint32_t lower = 0;
  uint32_t upper = map->size - 1;
  uint32_t i     = upper;
  int      cmp   = 0;

  while (upper >= lower) {
    i   = lower + ((upper - lower) / 2);
    cmp = strcmp(map->symbols[map->index[i] - 1], sym);

    if (cmp == 0) {
      *exact = true;
      return i;
    }

    if (cmp > 0) {
      if (i == 0) {
        break; // Avoid underflow
      }
      upper = i - 1;
    } else {
      lower = ++i;
    }
  }

  assert(!*exact || strcmp(map->symbols[map->index[i] - 1], sym) > 0);
  return i;
}

uint32_t
symap_try_map(Symap* map, const char* sym)
{
  bool           exact = false;
  const uint32_t index = symap_search(map, sym, &exact);
  if (exact) {
    assert(!strcmp(map->symbols[map->index[index]], sym));
    return map->index[index];
  }

  return 0;
}

uint32_t
symap_map(Symap* map, const char* sym)
{
  bool           exact = false;
  const uint32_t index = symap_search(map, sym, &exact);
  if (exact) {
    assert(!strcmp(map->symbols[map->index[index] - 1], sym));
    return map->index[index];
  }

  const uint32_t id  = ++map->size;
  char* const    str = symap_strdup(sym);

  // Append new symbol to symbols array
  map->symbols         = (char**)realloc(map->symbols, map->size * sizeof(str));
  map->symbols[id - 1] = str;

  // Insert new index element into sorted index
  map->index = (uint32_t*)realloc(map->index, map->size * sizeof(uint32_t));
  if (index < map->size - 1) {
    memmove(map->index + index + 1,
            map->index + index,
            (map->size - index - 1) * sizeof(uint32_t));
  }

  map->index[index] = id;

  return id;
}

const char*
symap_unmap(Symap* map, uint32_t id)
{
  if (id == 0) {
    return NULL;
  }

  if (id <= map->size) {
    return map->symbols[id - 1];
  }

  return NULL;
}

#ifdef STANDALONE

#  include <stdio.h>

static void
symap_dump(Symap* map)
{
  fprintf(stderr, "{\n");
  for (uint32_t i = 0; i < map->size; ++i) {
    fprintf(
      stderr, "\t%u = %s\n", map->index[i], map->symbols[map->index[i] - 1]);
  }
  fprintf(stderr, "}\n");
}

int
main()
{
#  define N_SYMS 5
  char* syms[N_SYMS] = {"hello", "bonjour", "goodbye", "aloha", "salut"};

  Symap* map = symap_new();
  for (int i = 0; i < N_SYMS; ++i) {
    if (symap_try_map(map, syms[i])) {
      fprintf(stderr, "error: Symbol already mapped\n");
      return 1;
    }

    const uint32_t id = symap_map(map, syms[i]);
    if (strcmp(map->symbols[id - 1], syms[i])) {
      fprintf(stderr, "error: Corrupt symbol table\n");
      return 1;
    }

    if (symap_map(map, syms[i]) != id) {
      fprintf(stderr, "error: Remapped symbol to a different ID\n");
      return 1;
    }

    symap_dump(map);
  }

  symap_free(map);
  return 0;
}

#endif // STANDALONE