126 lines
2.7 KiB
Lua
126 lines
2.7 KiB
Lua
-- see if the file exists
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local function file_exists(file)
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local f = io.open(file, "rb")
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if f then
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f:close()
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end
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return f ~= nil
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end
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-- get all lines from a file, returns an empty
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-- list/table if the file does not exist
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local function lines_from(file)
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if not file_exists(file) then
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return {}
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end
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local lines = {}
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for line in io.lines(file) do
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lines[#lines + 1] = line
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end
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return lines
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end
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local function splitstr(inputstr, sep)
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if sep == nil then
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sep = "%s"
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end
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local t = {}
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for str in string.gmatch(inputstr, "([^" .. sep .. "]+)") do
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t[#t + 1] = str
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end
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return t
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end
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-- tests the functions above
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local file = "input"
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local lines = lines_from(file)
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local reports = {}
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for _, line in ipairs(lines) do
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local raw_report = splitstr(line)
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for i, v in ipairs(raw_report) do
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raw_report[i] = tonumber(v)
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end
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reports[#reports + 1] = raw_report
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end
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-- return the sign of a number
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local function sign(number)
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if number > 0 then
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return 1
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elseif number == 0 then
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return 0
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else
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return -1
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end
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end
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function table.shallow_copy(t)
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local t2 = {}
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for k, v in pairs(t) do
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t2[k] = v
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end
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return t2
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end
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local function is_safe(report)
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local s = nil -- keeps track of the sign over the course of a report
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local unsafe = false
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local unsafe_idx_1, unsafe_idx_2
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for i, v1 in ipairs(report) do
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local v2 = report[i + 1]
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-- if v2 doesn't exist, we are at the end of the report
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if v2 ~= nil then
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if s == nil then
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-- if we are on the first iteration, we need to set the direction for
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-- the rest of the report
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s = sign(v1 - v2)
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else
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-- we check that the sign matches the rest of the report, breaking early if not
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if sign(v1 - v2) ~= s then
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unsafe = true
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unsafe_idx_1, unsafe_idx_2 = i, i + 1
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break
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end
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end
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-- Checking if the rate of change in the report is within acceptable bounds
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local distance = math.abs(v1 - v2)
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if distance < 1 or distance > 3 then
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unsafe = true
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unsafe_idx_1, unsafe_idx_2 = i, i + 1
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break
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end
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end
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end
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return unsafe, unsafe_idx_1, unsafe_idx_2
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end
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-- Copy the report, remove the specified index, and test if it is safe
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local function test_altered_report(report, idx)
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local test_report = table.shallow_copy(report)
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table.remove(test_report, idx)
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return is_safe(test_report)
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end
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local safe_count = 0
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for _, report in ipairs(reports) do
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-- if unsafe == true, idx1 and idx2 will be null
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local unsafe, idx1, idx2 = is_safe(report)
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if unsafe then
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-- if the report is unsafe, generate 2 reports with the potentially bad values removed, and test them again
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if not test_altered_report(report, idx1) or not test_altered_report(report, idx2) then
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unsafe = false
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end
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end
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if not unsafe then
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safe_count = safe_count + 1
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end
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end
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print(safe_count)
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