# Copyright 2012 Google Inc.
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
#
# * Redistributions of source code must retain the above copyright
#   notice, this list of conditions and the following disclaimer.
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#   notice, this list of conditions and the following disclaimer in the
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#   may be used to endorse or promote products derived from this software
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#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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# \file config.subr
# Configuration file processing and queries.

shtk_import cli


# List of valid configuration variables.
#
# This is initialized by shtk_config_init and should remain unchanged thorough the
# execution of the program.
_Shtk_ConfigVars=


# List of overrides to apply by shtk_config_apply_overrides.
#
# The overrides are recorded and applied separately because they need to happen
# after a configuration file has been loaded.  The contents of this list are
# words of the form: set:<variable> or unset:<variable>.  For those variables
# listed in set:, there is a corresponding shtk_config_override_var_<variable>
# variable containing the new value.
_Shtk_ConfigOverrides=


# Initializes the configuration module.
#
# \param ... List of configuration variables to recognize in configuration files
#     and user overrides.
shtk_config_init() {
    _Shtk_ConfigVars="${@}"
}


# Checks if a configuration variable is known.
#
# \param var Name of the variable to check.
#
# \return True if the variable was registered by shtk_config_init, false otherwise.
shtk_config_is_valid() {
    local var="${1}"; shift

    local known_var
    for known_var in ${_Shtk_ConfigVars}; do
        if [ "${known_var}" = "${var}" ]; then
            return 0
        fi
    done
    return 1
}


# Checks if a configuration variable is defined.
#
# \param var The name of the variable to check.
#
# \return True if the variable is defined (even if empty), false otherwise.
shtk_config_has() {
    local var="${1}"; shift

    local is_unset
    eval is_unset="\${shtk_config_var_${var}-yes_this_is_unset}"
    if [ "${is_unset}" = yes_this_is_unset ]; then
        return 1
    else
        return 0
    fi
}


# Gets the value of a defined configuration variable.
#
# \post The value of the variable is printed to stdout, if any.
#
# \post If the variable is not defined, this terminates execution with an error.
#
# \param var Name of the configuration variable to query.
shtk_config_get() {
    local var="${1}"; shift

    if shtk_config_has "${var}"; then
        eval echo "\${shtk_config_var_${var}}"
    else
        shtk_cli_error "Required configuration variable ${var} not set"
    fi
}


# Gets the value of configuration variable interpreting it as a boolean.
#
# \param var The variable to query.
#
# \return True if the variable is set to a truth value, false if its value is
# false or if it is not defined.
shtk_config_get_bool() {
    local var="${1}"; shift

    if shtk_config_has "${var}"; then
        case "$(shtk_config_get "${var}")" in
            [Yy][Ee][Ss]|[Tt][Rr][Uu][Ee])
                return 0
                ;;
            [Nn][Oo]|[Ff][Aa][Ll][Ss][Ee])
                return 1
                ;;
            *)
                shtk_cli_error "Invalid boolean value in variable ${var}"
                ;;
        esac
    else
        return 1
    fi
}


# Gets the value of a configuration variable with a default fallback.
#
# \post The value of the variable is printed to stdout, if any.
#
# \param var Name of the configuration variable to query.
# \param default Default value to return if the variable is not defined.
shtk_config_get_default() {
    local var="${1}"; shift
    local default="${1}"; shift

    if shtk_config_has "${var}"; then
        shtk_config_get "${var}"
    else
        echo "${default}"
    fi
}


# Sets a configuration variable.
#
# \post Execution terminates if the variable is not valid.
#
# \param var The name of the configuration variable to set.
# \param value The value to set.
shtk_config_set() {
    local var="${1}"; shift
    local value="${1}"; shift

    shtk_config_is_valid "${var}" \
        || shtk_cli_usage_error "Unknown configuration variable ${var}"
    eval "shtk_config_var_${var}=\"\${value}\""
}


# Unsets a configuration variable.
#
# \param var The name of the configuration variable to unset.
shtk_config_unset() {
    local var="${1}"; shift

    shtk_config_is_valid "${var}" \
        || shtk_cli_usage_error "Unknown configuration variable ${var}"
    eval unset "shtk_config_var_${var}"
}


# Loads a configuration file.
#
# \pre shtk_config_init should have been called to register the valid configuration
# variables.  Any non-registered configuration variable found in the file will
# not be available through any of the functions in this module.
#
# \post The configuration module is updated with the values defined in the
# configuration file.
#
# \post Any errors in the processing of the configuration file terminate the
# execution of the script.
#
# \param config_file Path to the file to load.
shtk_config_load() {
    local config_file="${1}"; shift

    [ -e "${config_file}" ] || shtk_cli_error "Configuration file" \
        "${config_file} does not exist"

    # User-facing variables.
    local var
    for var in ${_Shtk_ConfigVars}; do
        unset "${var}" || true
        eval local "${var}"
    done

    local real_config_file
    case "${config_file}" in
        */*) real_config_file="${config_file}" ;;
        *) real_config_file="./${config_file}" ;;
    esac
    ( . "${real_config_file}" ) || shtk_cli_error "Failed to load" \
        "configuration file ${config_file}"
    . "${real_config_file}"

    for var in ${_Shtk_ConfigVars}; do
        local value
        eval value="\${${var}-unset}"
        [ "${value-unset}" != unset ] || continue

        if [ -n "${value}" ]; then
            shtk_config_set "${var}" "${value}"
        else
            unset "shtk_config_var_${var}" || true
        fi
    done

    shtk_config_apply_overrides
}


# Applies recorded overrides to the current configuration.
#
# This is just a helper function for shtk_config_load and should not be used
# directly.
#
# \post The configuration data in memory is modified according to the data
# recorded in the overrides.
#
# \post The contents of _Shtk_ConfigOverrides is cleared.
shtk_config_apply_overrides() {
    for override in ${_Shtk_ConfigOverrides}; do
        case "${override}" in
            set:*)
                local var="$(echo ${override} | cut -d : -f 2)"
                local value
                eval value="\"\${shtk_config_override_var_${var}}\""
                shtk_config_set "${var}" "${value}"
                ;;
            unset:*)
                local var="$(echo ${override} | cut -d : -f 2)"
                unset "shtk_config_var_${var}" || true
                ;;
        esac
    done
    _Shtk_ConfigOverrides=
}


# Records an override to be applied to the configuration.
#
# Overrides are configuration variables set through the command line.  These can
# be set before loading the configuration file with shtk_config_load.
#
# \post Any errors in the processing of the configuration override terminate the
# execution of the script.
#
# \param arg An override of the form variable=value.
shtk_config_override() {
    local arg="${1}"; shift

    case "${arg}" in
        *=*)
            ;;
        *)
            shtk_cli_usage_error "Invalid configuration override" \
                "${arg}; must be of the form variable=value"
            ;;
    esac
    local var="$(echo "${arg}" | cut -d = -f 1)"
    local value="$(echo "${arg}" | cut -d = -f 2-)"

    [ -n "${var}" ] || shtk_cli_usage_error "Invalid configuration override" \
        "${arg}; must be of the form variable=value"
    shtk_config_is_valid "${var}" \
        || shtk_cli_usage_error "Unknown configuration variable ${var}"

    if [ -n "${value}" ]; then
        eval "shtk_config_override_var_${var}=\"${value}\""
        _Shtk_ConfigOverrides="${_Shtk_ConfigOverrides} set:${var}"
    else
        _Shtk_ConfigOverrides="${_Shtk_ConfigOverrides} unset:${var}"
    fi
}