2020 day 14

2020/13/day13.lua

@@ -0,0 +1,29 @@
+local start_timestamp = tonumber(io.read('l'))
+local buses = {}
+
+-- 'x' is translated to -1
+io.read('l'):gsub('[^,]*', function (bus) table.insert(buses, tonumber(bus) or -1) end)
+
+-- Copy and sort the buses, ignoring 'x' for part 1
+local sorted_buses = {}
+for _, bus in ipairs(buses) do
+    if bus > 0 then
+        table.insert(sorted_buses, bus)
+    end
+end
+table.sort(sorted_buses)
+
+local function find_bus(timestamp)
+    for _, bus in ipairs(sorted_buses) do
+        if bus > 0 and timestamp % bus == 0 then
+            return bus
+        end
+    end
+end
+
+local timestamp, bus_id = start_timestamp - 1, nil
+repeat
+    timestamp = timestamp + 1
+    bus_id = find_bus(timestamp)
+until bus_id
+print(bus_id * (timestamp - start_timestamp))

2020/14/day14.lua

@@ -0,0 +1,82 @@
+local instructions = {}
+
+for line in io.lines() do
+    local name, value = line:match('^(.+) = ([X%d]+)$')
+    table.insert(instructions, {name, value})
+end
+
+local function run(func)
+    local context = {mem={}}
+    for _, instruction in ipairs(instructions) do
+        context = func(context, table.unpack(instruction))
+    end
+
+    local sum = 0
+    for _, value in pairs(context.mem) do
+        sum = sum + value
+    end
+    return sum
+end
+
+local function part1(context, name, value)
+    if name == 'mask' then
+        context.and_mask = tonumber(value:gsub('X', '1'), 2)
+        context.or_mask = tonumber(value:gsub('X', '0'), 2)
+    else
+        local address = tonumber(name:match('%[(%d+)%]'))
+        context.mem[address] = tonumber(value) & context.and_mask | context.or_mask
+    end
+    return context
+end
+
+-- Recursive method that sets both 0 and 1 versions of each X character in an address
+local function part2_set(context, base_address, value)
+    local found = nil
+    for i = 1, #base_address do
+        if base_address[i] == "X" then
+            found = i
+            break
+        end
+    end
+
+    if not found then
+        context.mem[tonumber(table.concat(base_address), 2)] = value
+        return
+    end
+
+    base_address[found] = '0'
+    part2_set(context, base_address, value)
+    base_address[found] = '1'
+    part2_set(context, base_address, value)
+    base_address[found] = 'X'
+end
+
+-- Binary representation of a decimal number
+local function dectobin(value)
+    local t = {}
+    for i = 36, 1, -1 do
+        t[i] = math.fmod(value, 2)
+        value = math.floor((value - t[i]) / 2)
+    end
+    return table.concat(t)
+end
+
+local function part2(context, name, value)
+    if name == 'mask' then
+        context.mask = value
+        return context
+    end
+
+    local base_address = dectobin(tonumber(name:match('%[(%d+)%]')))
+    local masked_address = {}
+    for i = 1, #context.mask do
+        local bit = context.mask:sub(i, i)
+        table.insert(masked_address, bit == "0" and base_address:sub(i, i) or bit)
+    end
+
+    part2_set(context, masked_address, value)
+    return context
+end
+
+print(run(part1))
+print(run(part2))

README.md

@@ -23,8 +23,8 @@ is acceptable; anything goes as long as I solve it myself!
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