Day 1

-- Single-line comment
--[[ Multi-line
     comment ]]

-- Variables are global by default
x = 10
name = "Lua"

-- Use 'local' for block-scoped variables (always prefer this)
local count = 0
local pi = 3.14159
local greeting = "Hello, " .. name  -- string concatenation with ..

-- nil means "no value"
local nothing = nil

-- Check types
print(type(x))          --> number
print(type(greeting))   --> string
print(type(nothing))    --> nil
print(type(true))       --> boolean
print(type(print))      --> function

-- Numbers are always floats internally in Lua 5.3+
-- (Lua 5.3+ has a distinct integer subtype)
print(type(10))         --> number
print(math.type(10))    --> integer
print(math.type(10.0))  --> float

-- if / elseif / else
local score = 85
if score >= 90 then
  print("A")
elseif score >= 80 then
  print("B")
else
  print("C or below")
end

-- Numeric for: from, to, [step]
for i = 1, 5 do
  io.write(i .. " ")   -- no newline
end
print()  -- newline

-- Count down
for i = 10, 1, -2 do
  io.write(i .. " ")
end
print()

-- while loop
local n = 1
while n <= 4 do
  n = n * 2
end
print(n)  --> 8

-- repeat / until (condition checked AFTER body)
local x = 0
repeat
  x = x + 1
until x >= 3
print(x)  --> 3

-- break exits any loop
for i = 1, 100 do
  if i > 5 then break end
  io.write(i .. " ")
end
print()

-- Standard function definition
local function greet(name)
  return "Hello, " .. name .. "!"
end
print(greet("world"))  --> Hello, world!

-- Functions are values — assign them
local add = function(a, b) return a + b end
print(add(3, 4))  --> 7

-- Multiple return values
local function minmax(t)
  local lo, hi = t[1], t[1]
  for _, v in ipairs(t) do
    if v < lo then lo = v end
    if v > hi then hi = v end
  end
  return lo, hi
end

local lo, hi = minmax({5, 2, 8, 1, 9, 3})
print(lo, hi)  --> 1    9

-- Varargs (variable number of arguments)
local function sum(...)
  local args = {...}  -- pack into table
  local total = 0
  for _, v in ipairs(args) do
    total = total + v
  end
  return total
end
print(sum(1, 2, 3, 4, 5))  --> 15

-- select('#', ...) returns argument count
local function count_args(...)
  return select('#', ...)
end
print(count_args(10, 20, 30))  --> 3

-- Counter factory — each call creates independent state
local function make_counter(start)
  local count = start or 0   -- 'count' is an upvalue
  return {
    inc  = function() count = count + 1 end,
    dec  = function() count = count - 1 end,
    get  = function() return count end,
  }
end

local c1 = make_counter(0)
local c2 = make_counter(100)

c1.inc(); c1.inc(); c1.inc()
c2.dec()

print(c1.get())  --> 3
print(c2.get())  --> 99

-- Closures capture by reference, not by value
local function make_adder(n)
  return function(x) return x + n end
end

local add5  = make_adder(5)
local add10 = make_adder(10)

print(add5(3))   --> 8
print(add10(3))  --> 13

local s = "Hello, Lua World!"

-- Length
print(#s)                          --> 17
print(string.len(s))               --> 17

-- Case
print(string.upper(s))             --> HELLO, LUA WORLD!
print(string.lower(s))             --> hello, lua world!

-- Substrings (1-indexed, negative from end)
print(string.sub(s, 1, 5))         --> Hello
print(string.sub(s, -6))           --> World!

-- Find (returns start, end positions)
local i, j = string.find(s, "Lua")
print(i, j)                        --> 8    10

-- Pattern matching with gmatch (iterate over matches)
local sentence = "the quick brown fox"
for word in string.gmatch(sentence, "%a+") do
  io.write("[" .. word .. "] ")
end
print()  --> [the] [quick] [brown] [fox]

-- gsub (global substitution)
local result, count = string.gsub(s, "%a+", function(w)
  return string.upper(w)
end)
print(result)  --> HELLO, LUA WORLD!
print(count)   --> 3

-- string.format (like printf)
local formatted = string.format("Pi = %.4f, Count = %d", math.pi, 42)
print(formatted)  --> Pi = 3.1416, Count = 42

-- Method syntax: s:upper() == string.upper(s)
print(s:sub(1, 5):upper())  --> HELLO