add inet:subnets()

Signed-off-by: Asbjørn Sloth Tønnesen <ast@2e8.dk>

3 files changed, 36 insertions, 1 deletions

diff --git a/README.rst b/README.rst
index 3bbb9c4..799f602 100644
--- a/README.rst
+++ b/README.rst
@@ -83,6 +83,7 @@ Operator          Description
 ``:hostmask()``   generate hostmask as an address
 ``:flip()``       flip the least significant network bit
 ``:bits()``       return the address bits in a table
+``:subnets()``    return the amount of /n subnets
 ================= ======================================
 
 
@@ -397,6 +398,24 @@ Valid values for ``n`` are ``1``, ``2``, ``4``, ``8``, ``16`` or ``32``.
     { 2, 0, 0, 1,  0, 13, 11, 8,   0, 0, 0, 0,  0, 0, 0, 0,
       0, 0, 0, 0,  0,  0,  0, 0,   0, 0, 0, 0,  0, 0, 4, 2 }
 
+
+``foo:subnets(n)``
+~~~~~~~~~~~~~~~~~~
+
+::
+
+  inet('192.0.2.0/24'):subnets(26) -- returns 4
+  inet('192.0.2.0/25'):subnets(26) -- returns 2
+  inet('192.0.2.0/26'):subnets(26) -- returns 1
+  inet('192.0.2.0/27'):subnets(26) -- returns 0.5
+  inet('192.0.2.0/28'):subnets(26) -- returns 0.25
+
+  inet('2001:db8::/48'):subnets(56) -- returns 256
+  inet('2001:db8::/56'):subnets(64) -- returns 256
+  inet('2001:db8::/48'):subnets(64) -- returns 65536
+  inet('2001:db8::/64'):subnets(56) -- returns 0.00390625
+
+
 Sets
 ----
 
diff --git a/lua/inet/core.lua b/lua/inet/core.lua
index f1c7c49..15dd0f9 100644
--- a/lua/inet/core.lua
+++ b/lua/inet/core.lua
@@ -6,6 +6,7 @@ local format = string.format
 local floor = math.floor
 local min = math.min
 local max = math.max
+local pow = math.pow
 local insert = table.insert
 
 local lshift = bit32.lshift
@@ -55,6 +56,16 @@ end
 inet4.__len = inet.__len
 inet6.__len = inet.__len
 
+function inet:subnets(n)
+	if type(n) ~= 'number' then return nil, 'n must be a number' end
+	local hostmask = is_inet6(self) and 128 or 32
+	if n < 0 or n > hostmask then return nil, 'invalid mask given' end
+	local mask = self.mask
+	local bits = n - mask
+	local subnets = pow(2, bits)
+	return subnets
+end
+
 function inet:family()
 	local mt = assert(getmetatable(self))
 	return assert(mt2fam[mt])
@@ -329,7 +340,7 @@ function inet4:__sub(n)
 end
 
 function inet4:__mul(n)
-	local new = self.bip + (n * math.pow(2, 32 - self.mask))
+	local new = self.bip + (n * pow(2, 32 - self.mask))
 	return new_inet4(new, self.mask)
 end
 
diff --git a/test/inet.lua b/test/inet.lua
index b974179..f2968bd 100644
--- a/test/inet.lua
+++ b/test/inet.lua
@@ -305,6 +305,11 @@ local function misc()
 	assert(inet('2001:db8::42/64'):bits(0) == nil)
 	assert(inet('2001:db8::42/64'):bits(42) == nil)
 	assert(inet('2001:db8::42/64'):bits(64) == nil)
+
+	assert(inet('192.0.2.24'):subnets(-1) == nil)
+	assert(inet('192.0.2.24'):subnets(33) == nil)
+	assert(inet('2001:db8::42/64'):subnets(-1) == nil)
+	assert(inet('2001:db8::42/64'):subnets(129) == nil)
 end
 
 local t = test.new()