#!/usr/bin/env bash

set -euo pipefail

usage() {
	cat >&2 <<EOF
Usage: $0 <function-name> <layer-name> [layer-version]
Updates or adds a Lambda layer while preserving existing layers.

Arguments:
function-name    Name of the Lambda function
layer-name       Name of the layer to add/update
layer-version    Layer version, defaults to latest
EOF
	exit 1
}

if [ $# -lt 2 ] || [ $# -gt 3 ]; then
	usage
fi

function_name="$1"
layer_name="$2"
version="${3:-}"
region=$(aws configure get region)
account_id="$AWS_ACCOUNT_ID"

# Get latest version if not specified
if [ -z "$version" ]; then
	version=$(aws lambda list-layer-versions \
		--layer-name "$layer_name" \
		--query 'max_by(LayerVersions, &Version).Version' \
		--output text)
fi

new_layer_arn="arn:aws:lambda:${region}:${account_id}:layer:${layer_name}:${version}"


# Get current layers and process them
layer_arns=()
while IFS= read -r layer_arn; do
	# Skip if it's the same layer (but different version)
	if [[ "$layer_arn" =~ ^(.+):([0-9]+)$ ]]; then
		base_arn="${BASH_REMATCH[1]}"
		if [[ "$new_layer_arn" =~ ^"$base_arn": ]]; then
			continue
		fi
	fi
	# Skip the update if the layer is already the right version
	if [ "$layer_arn" = "$new_layer_arn" ]; then
		echo "Layer $layer_name@$version is already attached to $function_name"
		exit 0
	fi
	layer_arns+=("$layer_arn")
done < <(aws lambda get-function --function-name "$function_name" --query 'Configuration.Layers[*].Arn' | jq -r '.[]')

# Add the new layer
layer_arns+=("$new_layer_arn")

# Update the function configuration
confirm aws lambda update-function-configuration --function-name "$function_name" --layers "${layer_arns[@]}"
aws lambda wait function-active --function-name "$function_name"