Functions for extracting values from a JSON string with pattern matching. This is faster than using dkjson when only a few fields are needed. License: WTFPL

json.lua

-- Extracts values from a JSON string with pattern matching
-- This is faster than using dkjson when only a few fields are needed

-- Use this only with trusted data sources! Limitations:
-- * Character escapes are not supported
-- * Field nesting is ignored

local find, gsub = string.find, string.gsub

---@param json string
---@param key string
---@param init number|nil
---@return string|nil, number|nil, number|nil
local function extractStringJsonValue (json, key, init)
  local pattern = [["]] .. key .. [["%s*:%s*"([^"]*)"]]
  local startIndex, endIndex, valueStr = find(json, pattern, init)
  return valueStr, startIndex, endIndex
end

---@param json string
---@param key string
---@param init number|nil
---@return number|nil, number|nil, number|nil
local function extractNumberJsonValue (json, key, init)
  local pattern = [["]] .. key .. [["%s*:%s*(-?[0-9.e-]+)]]
  local startIndex, endIndex, valueStr = find(json, pattern, init)
  return tonumber(valueStr), startIndex, endIndex
end

---@param json string
---@param key string
---@param init number|nil
---@return boolean|nil, number|nil, number|nil
local function extractBooleanJsonValue (json, key, init)
  local pattern = [["]] .. key .. [["%s*:%s*([truefals]+)]]
  local startIndex, endIndex, valueStr = find(json, pattern, init)

  if valueStr == "true" then
    return true, startIndex, endIndex
  elseif valueStr == "false" then
    return false, startIndex, endIndex
  else
    return nil
  end
end

---@param extractJsonValue function
---@param json string
---@param key string
---@param stopAfterIndex number|nil
---@param stopAfterValue any|nil
---@return any[]
local function extractAllJsonValues (extractJsonValue, json, key, stopAfterIndex, stopAfterValue)
  local values = {}
  local valuesLen = 0

  local jsonPos = 1
  local value, valueStartIndex, valueEndIndex -- luacheck: ignore valueStartIndex -- unused

  repeat
    value, valueStartIndex, valueEndIndex = extractJsonValue(json, key, jsonPos)

    if value ~= nil then
      valuesLen = valuesLen + 1
      values[valuesLen] = value

      jsonPos = valueEndIndex + 1
    end

    if value == stopAfterValue then break end
    if valuesLen == stopAfterIndex then break end
  until value == nil

  return values
end

---@param json string
---@param key string
---@param stopAfterIndex number|nil
---@param stopAfterValue string|nil
---@return string[]
local function extractAllStringJsonValues (json, key, stopAfterIndex, stopAfterValue)
  return extractAllJsonValues(extractStringJsonValue, json, key, stopAfterIndex, stopAfterValue)
end

---@param json string
---@param key string
---@param stopAfterIndex number|nil
---@param stopAfterValue number|nil
---@return number[]
local function extractAllNumberJsonValues (json, key, stopAfterIndex, stopAfterValue)
  return extractAllJsonValues(extractNumberJsonValue, json, key, stopAfterIndex, stopAfterValue)
end

---@param json string
---@param key string
---@param stopAfterIndex number|nil
---@param stopAfterValue boolean|nil
---@return boolean[]
local function extractAllBooleanJsonValues (json, key, stopAfterIndex, stopAfterValue)
  return extractAllJsonValues(extractBooleanJsonValue, json, key, stopAfterIndex, stopAfterValue)
end

---@param json string
---@param key string
---@return string
local function deleteAllStringJsonValues (json, key)
  local pattern = [[%s*"]] .. key .. [["%s*:%s*"[^"]*"%s*,?]]
  return (gsub(json, pattern, ""))
end

---@param json string
---@param key string
---@return string
local function deleteAllNumberJsonValues (json, key)
  local pattern = [[%s*"]] .. key .. [["%s*:%s*-?[0-9.e-]+%s*,?]]
  return (gsub(json, pattern, ""))
end

---@param json string
---@param key string
---@return string
local function deleteAllBooleanJsonValue (json, key)
  local pattern = [[%s*"]] .. key .. [["%s*:%s*[truefals]+%s*,?]]
  return (gsub(json, pattern, ""))
end

return {
  extractStringJsonValue = extractStringJsonValue,
  extractNumberJsonValue = extractNumberJsonValue,
  extractBooleanJsonValue = extractBooleanJsonValue,

  extractAllStringJsonValues = extractAllStringJsonValues,
  extractAllNumberJsonValues = extractAllNumberJsonValues,
  extractAllBooleanJsonValues = extractAllBooleanJsonValues,

  deleteAllStringJsonValues = deleteAllStringJsonValues,
  deleteAllNumberJsonValues = deleteAllNumberJsonValues,
  deleteAllBooleanJsonValue = deleteAllBooleanJsonValue
}