"""
Configuration Loader - Load YAML config with environment variable substitution
Compatible with Python 3.6+
"""

import os
import re
import yaml
from dotenv import load_dotenv

def load_config(config_path='config/config.yaml'):
    """
    Load configuration from YAML file with environment variable substitution

    Supports:
    - ${VAR_NAME} - Required environment variable
    - ${VAR_NAME:-default} - Optional with default value
    """
    # Load environment variables from .env file
    load_dotenv()

    # Read YAML file
    with open(config_path, 'r') as f:
        config_text = f.read()

    # Substitute environment variables
    config_text = substitute_env_vars(config_text)

    # Parse YAML
    config = yaml.safe_load(config_text)

    # Load environment-specific overrides if specified
    env = config.get('environment', 'staging')
    env_config_path = 'config/environments/{}.yaml'.format(env)

    if os.path.exists(env_config_path):
        with open(env_config_path, 'r') as f:
            env_config_text = f.read()
        env_config_text = substitute_env_vars(env_config_text)
        env_config = yaml.safe_load(env_config_text)

        # Merge environment-specific config
        config = deep_merge(config, env_config)

    return config

def substitute_env_vars(text):
    """
    Substitute ${VAR_NAME} and ${VAR_NAME:-default} patterns with environment variables
    """
    def replacer(match):
        var_expr = match.group(1)

        # Check for default value syntax: VAR:-default
        if ':-' in var_expr:
            var_name, default = var_expr.split(':-', 1)
            return os.getenv(var_name, default)
        else:
            # Required variable
            value = os.getenv(var_expr)
            if value is None:
                raise ValueError("Required environment variable not set: {}".format(var_expr))
            return value

    # Pattern: ${VAR_NAME} or ${VAR_NAME:-default}
    pattern = r'\$\{([^}]+)\}'
    return re.sub(pattern, replacer, text)

def deep_merge(base, override):
    """
    Deep merge two dictionaries
    """
    result = base.copy()

    for key, value in override.items():
        if key in result and isinstance(result[key], dict) and isinstance(value, dict):
            result[key] = deep_merge(result[key], value)
        else:
            result[key] = value

    return result

def load_field_mappings(config):
    """
    Load field mappings configuration
    """
    mappings_file = config['fields']['mappings_file']

    with open(mappings_file, 'r') as f:
        return yaml.safe_load(f)