/**
 * Simple USB Serial Echo Program with Timeout, Reconnect, and Backspace Logic.
 * This program now correctly handles the backspace character by deleting it 
 * from the buffer and sending the necessary control codes to the terminal 
 * for visual deletion.
 */

#include "pico/stdlib.h"
#include "pico/time.h" // Needed for time_us_64()
#include <stdio.h>
#include <ctype.h>

// Define the maximum size for the input string buffer
#define MAX_INPUT_LEN 64
// Define the timeout period: 1 second in microseconds
#define TIMEOUT_US 5000000
// ASCII code for Backspace (often sent as 0x08)
#define ASCII_BACKSPACE 8

// Helper function to handle the echoing and resetting of the buffer
void echo_and_reset(char* buffer, int* index_ptr) {
    if (*index_ptr > 0) {
        // Null-terminate the string
        buffer[*index_ptr] = '\0';
        
        // Echo the string back, converting to uppercase
        printf("\nEchoed (ALL CAPS): ");
        for (int i = 0; buffer[i] != '\0'; i++) {
            printf("%c", toupper(buffer[i]));
        }
        printf("\n");
    }
    
    // Reset buffer for next line and print a new prompt
    *index_ptr = 0;
    printf("> ");
}

// Function to handle the initial waiting and reconnect waiting
void wait_for_usb_connection() {
    printf("Waiting for USB host to connect...\n");
    // Wait until the USB host (e.g., terminal program) opens the serial port
    while (!stdio_usb_connected()) {
        sleep_ms(100);
    }
    // Connection established!
    printf("\nConnection Established! Starting Echo Session...\n");
}

void run_echo_session() {
    char input_buffer[MAX_INPUT_LEN];
    int buffer_index = 0;
    // Track the time (in microseconds) when the last character was received
    uint64_t last_char_time = time_us_64();
    
    printf("--- Pico USB String Echo Program Started ---\n");
    printf("Type a sentence (up to %d chars). It will echo on Enter/Timeout.\n", MAX_INPUT_LEN - 2);
    printf("--------------------------------------------\n");
    printf("> "); // Prompt for input

    // Inner loop runs ONLY while the USB connection is active
    while (stdio_usb_connected()) {
        
        // Check for incoming character without blocking
        int c = getchar_timeout_us(0); 

        if (c != PICO_ERROR_TIMEOUT) {
            
            // Character received, update the timer
            last_char_time = time_us_64();
            char input_char = (char)c;

            // 1. --- BACKSPACE HANDLING ---
            if (input_char == ASCII_BACKSPACE || input_char == 127) { // 127 is sometimes sent by terminals (DEL key)
                if (buffer_index > 0) {
                    // Decrease buffer index (remove the character)
                    buffer_index--; 
                    // Send backspace, space, and backspace to erase the character on the terminal:
                    // \b: move cursor back
                    // ' ': overwrite with a space
                    // \b: move cursor back again, ready for the next character
                    printf("\b \b"); 
                }
            // 2. --- End of Input (Newline/CR) ---
            } else if (input_char == '\r' || input_char == '\n') {
                echo_and_reset(input_buffer, &buffer_index);
            
            // 3. --- Collect regular characters ---
            } else if (buffer_index < (MAX_INPUT_LEN - 1) && isprint(input_char)) {
                
                // Echo character locally so the user sees what they type
                printf("%c", input_char);
                
                // Store character in the buffer
                input_buffer[buffer_index] = input_char;
                buffer_index++;
            }
            // Ignore non-printable characters and overflowed buffer input
        }
        
        // 4. Timeout Check 
        if (buffer_index > 0 && (time_us_64() - last_char_time) > TIMEOUT_US) {
            printf("\n--- Timeout Reached (5.0s silence) ---\n");
            echo_and_reset(input_buffer, &buffer_index);
            last_char_time = time_us_64(); // Reset time to prevent immediate re-trigger
        }
        
        // Add a small delay to prevent the loop from consuming too much CPU time
        sleep_us(100); 
    }

    // Connection lost or closed by host
    printf("\nHost disconnected. Ending Echo Session.\n");
}


int main() {
    // Initialize Standard I/O (stdio) to use the USB backend
    stdio_init_all();
    
    // Outer loop: Allows the program to run continuously and restart the
    // session after a disconnection.
    while (true) {
        
        // 1. Wait for a connection
        wait_for_usb_connection();

        // 2. Run the program logic while connected
        run_echo_session();
        
        // Loop repeats, going back to wait_for_usb_connection()
    }

    // Note: main should not return
}