#include "mcp/tools/common/registry.h"

#include "mcp/tools/common/common.h"
#include "mcp/tools/common/context.h"

#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include "base/allocator.h"
#include "storage/codec.h"

#define READ_DEFAULT_LIMIT 100
#define READ_MAX_LIMIT 1000
#define READ_RESPONSE_BUDGET (8U * 1024U * 1024U - 64U * 1024U)
#define CURSOR_BYTES 47U
#define CURSOR_TEXT_MAX 64U

typedef struct history_cursor
{
  int64_t timestamp;
  char message_id[27];
} history_cursor_t;

static uint64_t
session_hash(const char *value)
{
  uint64_t hash = UINT64_C(1469598103934665603);
  while (*value)
    {
      hash ^= (unsigned char)*value++;
      hash *= UINT64_C(1099511628211);
    }
  return hash;
}

static size_t
base64url_encode(const unsigned char *input, size_t length, char *output, size_t capacity)
{
  static const char alphabet[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
  size_t source = 0, target = 0;
  while (source < length)
    {
      uint32_t value = (uint32_t)input[source++] << 16;
      unsigned available = 1;
      if (source < length)
        {
          value |= (uint32_t)input[source++] << 8;
          available++;
        }
      if (source < length)
        {
          value |= input[source++];
          available++;
        }
      if (target + available + 1 >= capacity)
        {
          return 0;
        }
      output[target++] = alphabet[(value >> 18) & 63];
      output[target++] = alphabet[(value >> 12) & 63];
      if (available > 1)
        {
          output[target++] = alphabet[(value >> 6) & 63];
        }
      if (available > 2)
        {
          output[target++] = alphabet[value & 63];
        }
    }
  output[target] = '\0';
  return target;
}

static int
base64_value(unsigned char value)
{
  if (value >= 'A' && value <= 'Z')
    {
      return value - 'A';
    }
  if (value >= 'a' && value <= 'z')
    {
      return value - 'a' + 26;
    }
  if (value >= '0' && value <= '9')
    {
      return value - '0' + 52;
    }
  if (value == '-')
    {
      return 62;
    }
  if (value == '_')
    {
      return 63;
    }
  return -1;
}

static bool
base64url_decode(const char *input, unsigned char output[CURSOR_BYTES])
{
  size_t length = strlen(input), source = 0, target = 0;
  uint32_t accumulator = 0;
  unsigned bits = 0;
  if (length != 63)
    {
      return false;
    }
  while (source < length)
    {
      int value = base64_value((unsigned char)input[source++]);
      if (value < 0)
        {
          return false;
        }
      accumulator = (accumulator << 6) | (uint32_t)value;
      bits += 6;
      if (bits >= 8)
        {
          bits -= 8;
          if (target >= CURSOR_BYTES)
            {
              return false;
            }
          output[target++] = (unsigned char)(accumulator >> bits);
          accumulator &= bits ? (UINT32_C(1) << bits) - 1 : 0;
        }
    }
  return target == CURSOR_BYTES && accumulator == 0;
}

static bool
cursor_encode(const char *session, const tiny_message_t *message, char output[CURSOR_TEXT_MAX])
{
  unsigned char data[CURSOR_BYTES] = { 0 };
  data[0] = 1;
  tiny_codec_put_u64(data + 1, session_hash(session));
  tiny_codec_put_u64(data + 9, (uint64_t)message->timestamp_ms);
  memcpy(data + 17, message->message_id, 26);
  tiny_codec_put_u32(data + 43, tiny_codec_crc32(data, 43));
  return base64url_encode(data, sizeof(data), output, CURSOR_TEXT_MAX) == 63;
}

static bool
cursor_decode(const char *session, const char *value, history_cursor_t *cursor)
{
  unsigned char data[CURSOR_BYTES];
  if (!value || !value[0] || !base64url_decode(value, data) || data[0] != 1
      || tiny_codec_get_u64(data + 1) != session_hash(session)
      || tiny_codec_get_u32(data + 43) != tiny_codec_crc32(data, 43))
    {
      return false;
    }
  memcpy(cursor->message_id, data + 17, 26);
  cursor->message_id[26] = '\0';
  cursor->timestamp = (int64_t)tiny_codec_get_u64(data + 9);
  return cursor->timestamp >= 0 && tiny_message_id_valid(cursor->message_id);
}

static int
compare_cursor(const tiny_message_t *message, const history_cursor_t *cursor)
{
  if (message->timestamp_ms < cursor->timestamp)
    {
      return -1;
    }
  if (message->timestamp_ms > cursor->timestamp)
    {
      return 1;
    }
  return strcmp(message->message_id, cursor->message_id);
}

static size_t
escaped_size(const char *value, size_t length)
{
  size_t position, total = 0;
  for (position = 0; position < length; ++position)
    {
      unsigned char byte = (unsigned char)value[position];
      total += byte < 0x20 ? 6 : (byte == '"' || byte == '\\' ? 2 : 1);
    }
  return total;
}

static void
call_read_messages(const mcpc_tool_t *tool, mcpc_ucbr_t *result)
{
  tiny_mcp_tools_context_t *context = tiny_mcp_tools_context_current();
  char *session = tiny_mcp_tool_get_string(tool, "session_id", TINY_SESSION_ID_MAX);
  char *cursor_text = tiny_mcp_tool_get_string(tool, "cursor", CURSOR_TEXT_MAX);
  int32_t requested_limit = READ_DEFAULT_LIMIT;
  mcpc_errcode_t limit_status
      = mcpc_tool_get_tpropval_i32(tool, (const char8_t *)"limit", &requested_limit);
  const size_t *entries = NULL;
  size_t entry_count = 0, position = 0, emitted = 0, outer_size = 256;
  history_cursor_t cursor;
  bool has_cursor = cursor_text != NULL, more = false;
  const tiny_message_t *last = NULL;
  tiny_json_buffer_t json;

  if (!context || !session || !session[0]
      || (limit_status != MCPC_EC_0 && limit_status != MCPC_EC_ANYDATA_NOTFOUND)
      || requested_limit < 1 || requested_limit > READ_MAX_LIMIT
      || (has_cursor && !cursor_decode(session, cursor_text, &cursor)))
    {
      tiny_mcp_tool_add_error(result, "VALIDATION_ERROR", "invalid read_messages arguments");
      free(session);
      free(cursor_text);
      return;
    }
  if (!tiny_mcp_tools_ensure_history_index(context)
      || !tiny_history_index_session(context->history_index, session, &entries, &entry_count))
    {
      tiny_mcp_tool_add_error(result, "INTERNAL_ERROR", "history index allocation failed");
      free(session);
      free(cursor_text);
      return;
    }
  if (has_cursor)
    {
      size_t low = 0, high = entry_count;
      while (low < high)
        {
          size_t middle = low + (high - low) / 2;
          const tiny_store_entry_t *entry = tiny_store_entry(context->store, entries[middle]);
          if (compare_cursor(&entry->message, &cursor) <= 0)
            {
              low = middle + 1;
            }
          else
            {
              high = middle;
            }
        }
      position = low;
    }

  tiny_json_init(&json);
  tiny_json_raw(&json, "{\"history\":[");
  while (position < entry_count && emitted < (size_t)requested_limit)
    {
      const tiny_store_entry_t *entry = tiny_store_entry(context->store, entries[position]);
      tiny_json_buffer_t item;
      size_t item_size;
      if (!entry || entry->deleted)
        {
          position++;
          continue;
        }
      tiny_json_init(&item);
      tiny_mcp_tool_append_message(&item, &entry->message);
      item_size = escaped_size(item.data, item.length) + (emitted ? 2 : 0);
      if (item.failed || (emitted && outer_size + item_size > READ_RESPONSE_BUDGET))
        {
          tiny_json_free(&item);
          if (item.failed)
            {
              json.failed = true;
            }
          more = true;
          break;
        }
      if (emitted)
        {
          tiny_json_raw(&json, ",");
        }
      tiny_json_raw_n(&json, item.data, item.length);
      outer_size += item_size;
      emitted++;
      last = &entry->message;
      position++;
      tiny_json_free(&item);
    }
  if (!more)
    {
      size_t lookahead = position;
      while (lookahead < entry_count)
        {
          const tiny_store_entry_t *entry = tiny_store_entry(context->store, entries[lookahead++]);
          if (entry && !entry->deleted)
            {
              more = true;
              break;
            }
        }
    }
  tiny_json_raw(&json, "]");
  if (more && last)
    {
      char next_cursor[CURSOR_TEXT_MAX];
      if (!cursor_encode(session, last, next_cursor))
        {
          json.failed = true;
        }
      else
        {
          tiny_json_raw(&json, ",\"nextCursor\":");
          tiny_json_string(&json, next_cursor);
        }
    }
  tiny_json_raw(&json, "}");
  if (json.failed)
    {
      tiny_mcp_tool_add_error(result, "INTERNAL_ERROR", "response allocation failed");
    }
  else
    {
      tiny_mcp_tool_add_json(result, &json);
    }
  tiny_json_free(&json);
  free(session);
  free(cursor_text);
}

bool
tiny_mcp_register_read_messages(mcpc_server_t *server)
{
  mcpc_tool_t *tool = tiny_mcp_tool_new("read_messages", "Read a session page", call_read_messages);
  return tool && tiny_mcp_tool_add_property(tool, "session_id", "Session ID", MCPC_U8STR, true)
         && tiny_mcp_tool_add_property(
             tool, "limit", "Optional page limit from 1 to 1000; defaults to 100", MCPC_I32, false)
         && tiny_mcp_tool_add_property(
             tool, "cursor", "Optional opaque page cursor", MCPC_U8STR, false)
         && mcpc_server_add_tool(server, tool) == MCPC_EC_0;
}