from typing import TypedDict

# Holds the parsed information from each line.
class LineData(TypedDict):
  # Which character is being used for indentation. Avoids having to specify it on each terrace_parse_line call.
  indent: str
  # How many indent characters are present in the current line before the first non-indent character.
  level: int
  # The number of characters before the start of the line's "head" section.
  # (Normally the same as `level`)
  offsetHead: int
  # The number of characters before the start of the line's "tail" section.
  offsetTail: int

def createLineData(indent: str = ' ') -> LineData:
  """
  Initialize a LineData instance with default values.

  Parameters
  ----------
  indent : str
    The character to use for indenting lines. ONLY ONE CHARACTER IS CURRENTLY PERMITTED.
  Returns
  -------
  LineData
    A LineData dict with the specified indent character and all other values initialized to 0.
  """
  return { "indent": indent, "level": 0, "offsetHead": 0, "offsetTail": 0 }

def parseLine(line: str, lineData: LineData):
  """
  Core Terrace parser function, sets level, offsetHead, and offsetTail in a LineData object based on the current line.
  Note that this is a C-style function, lineData is treated as a reference and mutated in-place.

  Parameters
  ----------
  line : str
    A string containing a line to parse. Shouldn't end with a newline.
  lineData: LineData
    A LineData dict to store information about the current line in. **Mutated in-place!**
  """

  # Blank lines have no characters, the newline should be stripped off.
  # Special case handling for these allows them to be parsed quickly.
  if len(line) == 0:
    # Empty lines are treated as having the same level as the previous line, so lineData.line is not updated.
    lineData['offsetHead'] = 0
    lineData['offsetTail'] = 0
  else:
    # Count the number of indent characters in the current line.
    level = 0
    while level < len(line) and line[level] == lineData['indent']:
      level += 1
    lineData['level'] = level

    # Set offsetHead and offsetTail to level to start with.
    # offsetHead should always be equal to level, and offsetTail will always be equal to or greater than level.
    lineData['offsetHead'] = level
    lineData['offsetTail'] = level

    # Increment offsetTail until we encounter a space character (start of tail) or reach EOL (no tail present).
    while lineData['offsetTail'] < len(line) and line[lineData['offsetTail']] != ' ':
      lineData['offsetTail'] += 1