"""
app.py - Simple Unit Converter Web App using Gradio
---------------------------------------------------
Supports: Mass, Length, Volume, Temperature.
Beginner-friendly code with inline comments.

How to run in Google Colab:
1. !pip install -r requirements.txt
2. !python app.py
3. Click the "share" link generated by Gradio.
"""

import gradio as gr

# --- STEP 1: Define available units and conversion logic ---

# Conversion factors relative to a base unit for each category
UNIT_MAP = {
    "Mass": {       # base = grams
        "kg": 1000.0,
        "g": 1.0,
        "mg": 0.001,
        "lb": 453.592,
        "oz": 28.3495,
    },
    "Length": {     # base = meters
        "mm": 0.001,
        "cm": 0.01,
        "m": 1.0,
        "km": 1000.0,
        "in": 0.0254,
        "ft": 0.3048,
        "yd": 0.9144,
        "mi": 1609.34,
    },
    "Volume": {     # base = liters
        "mL": 0.001,
        "L": 1.0,
        "m³": 1000.0,
        "cup": 0.24,
        "pint": 0.47,
        "gallon": 3.785,
    },
}

# Special case: Temperature uses formulas (not multipliers)
TEMP_UNITS = ["C", "F", "K"]

# --- STEP 2: Conversion functions ---

def convert_temperature(value, from_u, to_u):
    """Convert temperature between Celsius, Fahrenheit, Kelvin."""
    # convert input to Celsius
    if from_u == "C":
        c = value
    elif from_u == "F":
        c = (value - 32) * 5/9
    elif from_u == "K":
        c = value - 273.15
    else:
        return "Unknown unit"

    # convert Celsius to target
    if to_u == "C":
        return c
    elif to_u == "F":
        return c * 9/5 + 32
    elif to_u == "K":
        return c + 273.15

def convert_linear(value, from_u, to_u, category):
    """Convert Mass, Length, Volume using base-unit multipliers."""
    mapping = UNIT_MAP[category]
    base_value = value * mapping[from_u]       # step 1: to base
    return base_value / mapping[to_u]          # step 2: to target

def convert(value, from_unit, to_unit, category):
    """Main function used by Gradio."""
    # --- Input validation ---
    try:
        val = float(value)
    except:
        return "❌ Please enter a numeric value."

    try:
        if category == "Temperature":
            result = convert_temperature(val, from_unit, to_unit)
        else:
            result = convert_linear(val, from_unit, to_unit, category)
    except Exception as e:
        return f"⚠️ Error: {e}"

    return f"{val} {from_unit} = {result:.4g} {to_unit}"

# --- STEP 3: Define unit lists for dropdown menus ---
UNITS_BY_CATEGORY = {
    "Mass": list(UNIT_MAP["Mass"].keys()),
    "Length": list(UNIT_MAP["Length"].keys()),
    "Volume": list(UNIT_MAP["Volume"].keys()),
    "Temperature": TEMP_UNITS,
}

# --- STEP 4: Build Gradio UI ---
with gr.Blocks() as demo:
    gr.Markdown("## 🔄 Simple Unit Converter")

    # Row: category + unit dropdowns
    with gr.Row():
        category = gr.Dropdown(list(UNITS_BY_CATEGORY.keys()), value="Mass", label="Category")
        from_unit = gr.Dropdown(UNITS_BY_CATEGORY["Mass"], label="From Unit")
        to_unit = gr.Dropdown(UNITS_BY_CATEGORY["Mass"], label="To Unit")

    # Input + button + output
    value_input = gr.Number(value=1, label="Value")
    convert_btn = gr.Button("Convert")
    output = gr.Textbox(label="Result")

    # --- Update dropdowns when category changes ---
    def update_units(cat):
        units = UNITS_BY_CATEGORY[cat]
        return gr.update(choices=units, value=units[0]), gr.update(choices=units, value=units[1])

    category.change(update_units, category, [from_unit, to_unit])
    convert_btn.click(convert, [value_input, from_unit, to_unit, category], output)

# Run app (share=True gives a public link in Colab)
if __name__ == "__main__":
    demo.launch(share=True)