Make several Lua helper functions handle nil

The Lua helper functions clamp(), has_prefix(), has_suffix(), trim(),
and split_string() now all return nil if they are called with nil
as (first) argument. This makes them more robust, for instance when
called on non-existent tags and so simplifies user code.

src/init.lua

@@ -14,10 +14,16 @@ local _define_table_impl = function(_type, _name, _columns, _options)
 end
 
 function osm2pgsql.has_prefix(str, prefix)
+    if str == nil then
+        return nil
+    end
     return str:sub(1, prefix:len()) == prefix
 end
 
 function osm2pgsql.has_suffix(str, suffix)
+    if str == nil then
+        return nil
+    end
     if suffix == '' then
         return true
     end
@@ -51,6 +57,9 @@ function osm2pgsql.way_member_ids(relation)
 end
 
 function osm2pgsql.clamp(value, low, high)
+    if value == nil then
+        return nil
+    end
     return math.min(math.max(value, low), high)
 end
 
@@ -118,6 +127,9 @@ end
 
 -- from http://lua-users.org/wiki/StringTrim
 function osm2pgsql.trim(str)
+    if str == nil then
+        return nil
+    end
     local from = str:match("^%s*()")
     return from > #str and "" or str:match(".*%S", from)
 end
@@ -142,11 +154,15 @@ function osm2pgsql.split_unit(str, default_unit)
 end
 
 function osm2pgsql.split_string(str, separator)
-    local pattern = '([^' .. (separator or ';') .. ']+)'
     local result = {}
-    for w in string.gmatch(str, pattern) do
-        result[#result + 1] = osm2pgsql.trim(w)
+
+    if str ~= nil then
+        local pattern = '([^' .. (separator or ';') .. ']+)'
+        for w in string.gmatch(str, pattern) do
+            result[#result + 1] = osm2pgsql.trim(w)
+        end
     end
+
     return result
 end
 

tests/lua/tests.lua

@@ -20,6 +20,7 @@ assert(o.has_prefix('addr:city', ''))
 assert(not o.has_prefix('name', 'addr:'))
 assert(o.has_prefix('a', 'a'))
 assert(not o.has_prefix('a', 'ab'))
+assert(o.has_prefix(nil, 'a') == nil)
 
 -- has_suffix()
 
@@ -29,6 +30,7 @@ assert(o.has_suffix('tiger:source', ''))
 assert(not o.has_suffix('name', ':source'))
 assert(o.has_suffix('a', 'a'))
 assert(not o.has_suffix('a', 'ba'))
+assert(o.has_suffix(nil, 'a') == nil)
 
 -- ---------------------------------------------------------------------------
 
@@ -39,6 +41,7 @@ do
     assert(o.clamp(3, -1, 1) == 1)
     assert(o.clamp(-3, -1, 1) == -1)
     assert(o.clamp(2.718, 0, 3.141) == 2.718)
+    assert(o.clamp(nil, -1, 1) == nil)
 end
 
 -- make_check_values_func without default
@@ -109,6 +112,7 @@ assert(osm2pgsql.trim(' a ') == 'a')
 assert(osm2pgsql.trim('  a  ') == 'a')
 assert(osm2pgsql.trim('  ab cd  ') == 'ab cd')
 assert(osm2pgsql.trim(' \t\r\n\f\va\000b \r\t\n\f\v') == 'a\000b')
+assert(osm2pgsql.trim(nil) == nil)
 
 -- split_unit
 
@@ -189,6 +193,9 @@ assert(r[1] == 'ab c;d')
 assert(r[2] == 'e f')
 assert(r[3] == 'ghi')
 
+r = o.split_string(nil)
+assert(#r == 0)
+
 -- ---------------------------------------------------------------------------
 
 print("All tests successful")