diff --git a/19/input.txt b/19/input.txt
new file mode 100644
index 0000000..806774f
--- /dev/null
+++ b/19/input.txt
@@ -0,0 +1,45 @@
+Al => ThF
+Al => ThRnFAr
+B => BCa
+B => TiB
+B => TiRnFAr
+Ca => CaCa
+Ca => PB
+Ca => PRnFAr
+Ca => SiRnFYFAr
+Ca => SiRnMgAr
+Ca => SiTh
+F => CaF
+F => PMg
+F => SiAl
+H => CRnAlAr
+H => CRnFYFYFAr
+H => CRnFYMgAr
+H => CRnMgYFAr
+H => HCa
+H => NRnFYFAr
+H => NRnMgAr
+H => NTh
+H => OB
+H => ORnFAr
+Mg => BF
+Mg => TiMg
+N => CRnFAr
+N => HSi
+O => CRnFYFAr
+O => CRnMgAr
+O => HP
+O => NRnFAr
+O => OTi
+P => CaP
+P => PTi
+P => SiRnFAr
+Si => CaSi
+Th => ThCa
+Ti => BP
+Ti => TiTi
+e => HF
+e => NAl
+e => OMg
+
+CRnCaSiRnBSiRnFArTiBPTiTiBFArPBCaSiThSiRnTiBPBPMgArCaSiRnTiMgArCaSiThCaSiRnFArRnSiRnFArTiTiBFArCaCaSiRnSiThCaCaSiRnMgArFYSiRnFYCaFArSiThCaSiThPBPTiMgArCaPRnSiAlArPBCaCaSiRnFYSiThCaRnFArArCaCaSiRnPBSiRnFArMgYCaCaCaCaSiThCaCaSiAlArCaCaSiRnPBSiAlArBCaCaCaCaSiThCaPBSiThPBPBCaSiRnFYFArSiThCaSiRnFArBCaCaSiRnFYFArSiThCaPBSiThCaSiRnPMgArRnFArPTiBCaPRnFArCaCaCaCaSiRnCaCaSiRnFYFArFArBCaSiThFArThSiThSiRnTiRnPMgArFArCaSiThCaPBCaSiRnBFArCaCaPRnCaCaPMgArSiRnFYFArCaSiThRnPBPMgAr
diff --git a/19/solve.rb b/19/solve.rb
new file mode 100644
index 0000000..e96c9e5
--- /dev/null
+++ b/19/solve.rb
@@ -0,0 +1,46 @@
+def get_molecules(replacements, molecule)
+  molecules = []
+  replacements.each do |pair|
+    index = 0
+    loop do
+      index = molecule.index(pair.first, index)
+      break unless index
+      molecules << molecule[0...index] + pair[1] + molecule[(index + pair[0].length)..-1]
+      index += pair[0].length
+    end
+  end
+  molecules
+end
+
+replacements = []
+medicine_molecule = ''
+File.readlines(ARGV[0]).each do |line|
+  if line.match(/^(\w+) => (\w+)$/)
+    replacements << Regexp.last_match[1..2]
+  else
+    medicine_molecule = line.strip unless /\S/ !~ line
+  end
+end
+
+# part 1
+puts "Part 1: #{get_molecules(replacements, medicine_molecule).uniq.length}"
+
+# part 2
+replacements.map! { |e| [e[1], e[0]] } # collapsing final molecule => 'e'
+steps = 0
+q = [[0, medicine_molecule]] # [steps, molecule]
+loop do
+  q.sort! { |a, b| a[1].length - b[1].length }
+  current = q.shift
+  get_molecules(replacements, current[1]).uniq.each do |mol|
+    if mol == 'e'
+      steps = current[0] + 1
+      break
+    else
+      q << [current[0] + 1, mol]
+    end
+  end
+  break if q.empty?
+  break unless steps == 0
+end
+puts "Part 2: #{steps}"
diff --git a/19/test_1.txt b/19/test_1.txt
new file mode 100644
index 0000000..1798c27
--- /dev/null
+++ b/19/test_1.txt
@@ -0,0 +1,5 @@
+H => HO
+H => OH
+O => HH
+
+HOH
diff --git a/19/test_2.txt b/19/test_2.txt
new file mode 100644
index 0000000..fdc20b0
--- /dev/null
+++ b/19/test_2.txt
@@ -0,0 +1,5 @@
+H => HO
+H => OH
+O => HH
+
+HOHOHO
diff --git a/19/test_3.txt b/19/test_3.txt
new file mode 100644
index 0000000..5075b42
--- /dev/null
+++ b/19/test_3.txt
@@ -0,0 +1,3 @@
+H => OO
+
+H2O
diff --git a/19/test_4.txt b/19/test_4.txt
new file mode 100644
index 0000000..70905c4
--- /dev/null
+++ b/19/test_4.txt
@@ -0,0 +1,7 @@
+e => H
+e => O
+H => HO
+H => OH
+O => HH
+
+HOHOHO