require "log"

# Simply invoking methods like `info` from the `Log` module uses the standard
# logger, which is already pre-configured for reasonable logging output to STDOUT.
Log.info { "standard logger" }

# Loggers can be configured with various settings to set their output format.
# We can change its severity to debug and add a timestamp, for example.
Log.setup(:debug, Log::IOBackend.new(formatter: Log::Formatter.new do |entry, io|
  io << entry.timestamp << " " << entry.severity << " -- " << entry.message
end))
Log.debug { "with timestamp" }

# It also supports emitting the file name and line from which the log method is called.
Log.setup(:debug, Log::IOBackend.new(formatter: Log::Formatter.new do |entry, io|
  io << entry.timestamp << " " << entry.severity << " -- " << entry.message << " (#{entry.source}:#{entry.line})"
end))
Log.debug { "with file/line" }

# It may be useful to create a custom logger. When creating a new logger,
# we can set a name to distinguish its output from other loggers.
mylog = Log.for("mylog")
mylog.info { "from mylog" }

# We can set the name on existing loggers as well.
mylog = Log.for("ohmy")
mylog.info { "from mylog"  }

# Loggers can have custom output targets; any IO works.
memory_io = IO::Memory.new
Log.setup(:info, Log::IOBackend.new(memory_io))
Log.info { "hello" }

# This will actually show it on standard output.
puts "from memory_io: #{memory_io.to_s}"

# Crystal doesn't have a built-in structured logging package like Go's slog,
# but we can create a simple JSON logger using the JSON module.
require "json"

class JSONLogger
  def initialize(@io : IO)
  end

  def info(message : String, **kwargs)
    log_entry = {
      "time" => Time.utc.to_s,
      "level" => "INFO",
      "msg" => message
    }.merge(kwargs)
    @io.puts(log_entry.to_json)
  end
end

json_logger = JSONLogger.new(STDOUT)
json_logger.info("hi there")

# In addition to the message, our JSON logger output can
# contain an arbitrary number of key-value pairs.
json_logger.info("hello again", key: "val", age: 25)