#!/bin/bash
    
usage() {
	echo "Usage: $0 [OPTIONS] <file1> ... [--stdin] ... <fileN> <template_file>"
	echo "This script populates a template file with content from other files and optionally standard input."
	echo ""
	echo "The position of the '--stdin' flag determines which placeholder will be filled"
	echo "by standard input. The placeholders are numbered sequentially starting from 1."
	echo ""
	echo "Arguments:"
	echo "  <file1>, ...         Input files whose content will be used to replace placeholders."
	echo "                       Placeholders are named %s (in the order they are provided)."
	echo "  --stdin              An optional flag. If present, the content from standard input"
	echo "                       will be used to replace the placeholder at its position."
	echo "  <template_file>      The template file containing the placeholders to be replaced."
	echo ""
	echo "Options:"
	echo "  -h, --help           Show this help message and exit."
	echo ""
	echo "Example with stdin as the second placeholder:"
	echo "  echo \"body content\" | ./template.sh nav.html --stdin foot.html template.html"
	echo "  (Here, the first %s will be nav.html, the second %s will be stdin, and the third %s will be foot.html)"
	echo ""
	echo "Example without stdin:"
	echo "  ./template.sh nav.html foot.html template.html"
	echo "  (Here, the first %s will be nav.html and the second %s will be foot.html)"
	exit 0
}

if [[ "$1" == "-h" || "$1" == "--help" ]]; then
	usage
fi

if [ "$#" -lt 2 ]; then
	echo "Error: Not enough arguments provided."
	usage
fi

# Initialize a variable to track the index
stdin_index=0
index=0

# Iterate through the positional parameters
for arg in "$@"; do
    index=$((index + 1))
    # Check if the current argument is "--stdin"
    if [[ "$arg" == "--stdin" ]]; then
        stdin_index=$index
        break
    fi
done

has_stdin=false
if [ "$stdin_index" -ne 0 ]; then
	has_stdin=true
	template_file="${!#}"

# Split arguments into files before and after the --stdin flag
files_before_stdin=("${@:1:$((stdin_index - 1))}")
files_after_stdin=("${@:$((stdin_index + 1)):$(($# - stdin_index - 1))}")

# Read from stdin
stdin_content=$(cat)
else
	# Without --stdin, all arguments except the last are input files
	template_file="${!#}"
	input_files=("${@:1:$(($# - 1))}")
fi

if [ ! -f "$template_file" ]; then
	echo "Error: Template file '$template_file' not found."
	exit 1
fi

printf_args=()

for file in "${files_before_stdin[@]}"; do
	if [ -f "$file" ]; then
		printf_args+=("$(cat "$file")")
	else
		echo "Warning: File '$file' not found."
		printf_args+=("") # Add an empty string to maintain placeholder count
	fi
done

if [ "$has_stdin" = true ]; then
	printf_args+=("$stdin_content")
fi

for file in "${files_after_stdin[@]}"; do
	if [ -f "$file" ]; then
		printf_args+=("$(cat "$file")")
	else
		echo "Warning: File '$file' not found."
		printf_args+=("") # Add an empty string to maintain placeholder count
	fi
done

template_content=$(<"$template_file")

format_string=$(echo "$template_content" | sed -r 's/\$V[0-9]+/%s/g')

printf -- "$format_string" "${printf_args[@]}"