"""Helper functions for decoding event messages."""

import re

# Regex to match spaces where we need to embed params in the event message templates,

# e.g. {p0}, {p1+2}, {p3:dict}, {p4+1|dict}

EMBEDDED_PARAM_RE = re.compile(r"\{p(\d+)(?:\+(\d+))?(?::[\w]+)?(?:\|(\w+))?\}")

def render_event_template(

event_description_template: str, params_bytes: list, msg_json_data: dict

) -> str:

"""

Produce an event message string by replacing placeholders with parameter values.

Example template:

"Event {p0} occurred with value {p1+2|dictName}"

This would replace {p0} with the hex value of params[0], and replace {p1+2:dictName}

with the combined hex value of params[1] and params[2] (treated as big-endian bytes)

looked up in the dictionary named "dictName" for a human-readable string, or

rendered as hex if not found in the dictionary.

Parameters

----------

event_description_template : str

The event message template containing placeholders like {p0}, {p1+2},

{p3:dict}, {p4+1|dict}.

params_bytes : list

The list of parameter values to substitute into the template.

msg_json_data : dict

Mapping of parameter values to human-readable strings for decoding event

messages.

Returns

-------

str

The rendered event message with placeholders replaced by parameter values.

"""

def replace(m: re.Match[str]) -> str:

"""

Replace a single placeholder match with the corresponding parameter value.

Parameters

----------

m : re.Match[str]

A regex match object for a placeholder in the template.

Returns

-------

str

The string to replace the placeholder with.

"""

# We are parsing the placeholder value here to determine which parameter

# to substitute and how to format them.

# group one is the parameter index e.g. p0-3

idx = int(m.group(1))

# group two is the optional byte length e.g. +2 (so we know how many params

# to combine)

n_bytes = int(m.group(2)) if m.group(2) else 1

# group three is an optional dictionary name to use for decoding this parameter

dict_name = m.group(3)

value = 0

for i in range(n_bytes):

# combine the next n_bytes params into a single integer value, treating

# them as big-endian bytes

value = (value << 8) | (

params_bytes[idx + i] if idx + i < len(params_bytes) else 0

)

# If a dictionary name is provided use it to decode the value, otherwise just

# render as hex.

if dict_name:

resolved = msg_json_data.get(dict_name, {}).get(

value, f"0x{value:0{2 * n_bytes}X}"

)

return f"{dict_name}({resolved})" # wrap with dict name

return f"{value:02x}"

# Replace all placeholders in the template using the replace function defined above.

return EMBEDDED_PARAM_RE.sub(replace, event_description_template)